tighten up loop test
tighten up loop test

This trims one instruction of loop overhead, from

   sub(N, K)    // N -= 8
   cmp(N, K-1)  // if (N >= 8)
   jg loop      //     goto loop;

to

   sub(N, K)    // N -= 8
   ...          // if (N != 0)
   jne loop     //     goto loop;

To make this work we pass only multiples of K into the
JIT'd code, so it always hits exactly N = 0 to exit.

Change-Id: I81c7a2f8d5927971059a68c5bce2e3d6fb0b8ff2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/220576
Reviewed-by: Herb Derby <9e12a3d2bbf73546e47cc9958f6ba8f3215b7eaf@google.com>
Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>

aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,google/skia,google/skia,google/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,HalCanary/skia-hc,google/skia,aosp-mirror/platform_external_skia,google/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,HalCanary/skia-hc,HalCanary/skia-hc,HalCanary/skia-hc,HalCanary/skia-hc,google/skia,google/skia,HalCanary/skia-hc