simplify logic in SkPerliNoiseShader for GPU when constant color
simplify logic in SkPerliNoiseShader for GPU when constant color

BUG=skia:3488

Review URL: https://codereview.chromium.org/974183002

AOSPB/external_skia,noselhq/skia,google/skia,w3nd1go/android_external_skia,Infinitive-OS/platform_external_skia,ominux/skia,boulzordev/android_external_skia,noselhq/skia,Infinitive-OS/platform_external_skia,google/skia,timduru/platform-external-skia,todotodoo/skia,pcwalton/skia,OneRom/external_skia,Hikari-no-Tenshi/android_external_skia,Jichao/skia,OneRom/external_skia,VRToxin-AOSP/android_external_skia,MonkeyZZZZ/platform_external_skia,HalCanary/skia-hc,nvoron23/skia,YUPlayGodDev/platform_external_skia,YUPlayGodDev/platform_external_skia,noselhq/skia,MarshedOut/android_external_skia,vanish87/skia,aosp-mirror/platform_external_skia,YUPlayGodDev/platform_external_skia,todotodoo/skia,todotodoo/skia,AOSP-YU/platform_external_skia,amyvmiwei/skia,nfxosp/platform_external_skia,tmpvar/skia.cc,MinimalOS-AOSP/platform_external_skia,PAC-ROM/android_external_skia,amyvmiwei/skia,AOSP-YU/platform_external_skia,tmpvar/skia.cc,VRToxin-AOSP/android_external_skia,OneRom/external_skia,noselhq/skia,HalCanary/skia-hc,shahrzadmn/skia,TeamTwisted/external_skia,Infinitive-OS/platform_external_skia,pcwalton/skia,Infinitive-OS/platform_external_skia,VRToxin-AOSP/android_external_skia,VRToxin-AOSP/android_external_skia,google/skia,TeamExodus/external_skia,tmpvar/skia.cc,OneRom/external_skia,TeamTwisted/external_skia,boulzordev/android_external_skia,MonkeyZZZZ/platform_external_skia,AOSP-YU/platform_external_skia,Jichao/skia,noselhq/skia,YUPlayGodDev/platform_external_skia,AOSPB/external_skia,Jichao/skia,Hikari-no-Tenshi/android_external_skia,ominux/skia,Hikari-no-Tenshi/android_external_skia,TeamTwisted/external_skia,HalCanary/skia-hc,rubenvb/skia,BrokenROM/external_skia,todotodoo/skia,MinimalOS-AOSP/platform_external_skia,invisiblek/android_external_skia,MarshedOut/android_external_skia,amyvmiwei/skia,amyvmiwei/skia,nfxosp/platform_external_skia,nvoron23/skia,ominux/skia,aosp-mirror/platform_external_skia,UBERMALLOW/external_skia,MarshedOut/android_external_skia,geekboxzone/mmallow_external_skia,TeamTwisted/external_skia,MinimalOS-AOSP/platform_external_skia,HalCanary/skia-hc,geekboxzone/mmallow_external_skia,shahrzadmn/skia,YUPlayGodDev/platform_external_skia,qrealka/skia-hc,AOSPB/external_skia,vanish87/skia,aosp-mirror/platform_external_skia,nfxosp/platform_external_skia,MarshedOut/android_external_skia,vanish87/skia,geekboxzone/mmallow_external_skia,PAC-ROM/android_external_skia,HalCanary/skia-hc,google/skia,UBERMALLOW/external_skia,w3nd1go/android_external_skia,TeamTwisted/external_skia,shahrzadmn/skia,MinimalOS-AOSP/platform_external_skia,TeamExodus/external_skia,geekboxzone/mmallow_external_skia,nfxosp/platform_external_skia,HalCanary/skia-hc,shahrzadmn/skia,AOSPB/external_skia,pcwalton/skia,noselhq/skia,Jichao/skia,aosp-mirror/platform_external_skia,Infinitive-OS/platform_external_skia,Hikari-no-Tenshi/android_external_skia,w3nd1go/android_external_skia,HalCanary/skia-hc,spezi77/android_external_skia,geekboxzone/mmallow_external_skia,ominux/skia,rubenvb/skia,OneRom/external_skia,AOSPB/external_skia,rubenvb/skia,nfxosp/platform_external_skia,boulzordev/android_external_skia,TeamExodus/external_skia,PAC-ROM/android_external_skia,AOSP-YU/platform_external_skia,YUPlayGodDev/platform_external_skia,amyvmiwei/skia,MonkeyZZZZ/platform_external_skia,qrealka/skia-hc,pcwalton/skia,google/skia,TeamExodus/external_skia,pcwalton/skia,rubenvb/skia,rubenvb/skia,BrokenROM/external_skia,UBERMALLOW/external_skia,Infinitive-OS/platform_external_skia,TeamExodus/external_skia,TeamExodus/external_skia,BrokenROM/external_skia,timduru/platform-external-skia,MarshedOut/android_external_skia,timduru/platform-external-skia,tmpvar/skia.cc,BrokenROM/external_skia,Infinitive-OS/platform_external_skia,vanish87/skia,noselhq/skia,nvoron23/skia,UBERMALLOW/external_skia,OneRom/external_skia,aosp-mirror/platform_external_skia,Jichao/skia,todotodoo/skia,BrokenROM/external_skia,todotodoo/skia,MinimalOS-AOSP/platform_external_skia,PAC-ROM/android_external_skia,PAC-ROM/android_external_skia,Jichao/skia,MonkeyZZZZ/platform_external_skia,VRToxin-AOSP/android_external_skia,vanish87/skia,vanish87/skia,YUPlayGodDev/platform_external_skia,MinimalOS-AOSP/platform_external_skia,Hikari-no-Tenshi/android_external_skia,shahrzadmn/skia,Hikari-no-Tenshi/android_external_skia,invisiblek/android_external_skia,MarshedOut/android_external_skia,rubenvb/skia,HalCanary/skia-hc,AOSP-YU/platform_external_skia,noselhq/skia,Infinitive-OS/platform_external_skia,google/skia,w3nd1go/android_external_skia,tmpvar/skia.cc,invisiblek/android_external_skia,amyvmiwei/skia,todotodoo/skia,UBERMALLOW/external_skia,OneRom/external_skia,MonkeyZZZZ/platform_external_skia,PAC-ROM/android_external_skia,nfxosp/platform_external_skia,UBERMALLOW/external_skia,YUPlayGodDev/platform_external_skia,invisiblek/android_external_skia,HalCanary/skia-hc,AOSP-YU/platform_external_skia,ominux/skia,BrokenROM/external_skia,boulzordev/android_external_skia,TeamExodus/external_skia,timduru/platform-external-skia,pcwalton/s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