cmd/5g: avoid temporary during OMINUS
Saves one MOVW and one register during the fast div/mod introduced in CL
6819123.
linux/arm (armv5)
benchmark old ns/op new ns/op delta
BenchmarkInt64Mod1 12 12 +7.50%
BenchmarkUint16Mod2 7 7 +0.28%
BenchmarkUint16Mod4 7 7 -0.28%
BenchmarkUint64Mod1 15 11 -23.72%
BenchmarkInt8Neg 8 7 -17.66%
BenchmarkInt16Neg 8 7 -17.66%
BenchmarkInt32Neg 5 5 -9.04%
BenchmarkUint8Neg 7 6 -14.35%
BenchmarkUint16Neg 8 7 -17.66%
BenchmarkUint32Neg 5 5 -9.04%
R=rsc
CC=golang-dev
https://golang.org/cl/
6842045