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

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