cmd/5g: improve shift code generation
cmd/5g: improve shift code generation

This CL is a backport of 6012049 which improves code
generation for shift operations.

benchmark       old ns/op    new ns/op    delta
BenchmarkLSL            9            5  -49.67%
BenchmarkLSR            9            4  -50.00%

R=golang-dev, minux.ma, r, rsc
CC=golang-dev
http://codereview.appspot.com/6813045

tv42/old-go,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,gnanderson/go,tv42/old-go,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,dsriram/golang,apparentlymart/golang,gnanderson/go,apparentlymart/golang,gnanderson/go,dsriram/golang,gnanderson/go