in 13416:67c0b8c8fb29 "faster code, mainly for rotate" [1]. The codegen
can run out of registers if there are too many small-int arithmetic ops.

An alternative approach is to copy 6g's sbop/abop codegen to 8g, but
this change is less risky.

Fixes #3835.

[1] http://code.google.com/p/go/source/diff?spec=svn67c0b8c8fb29b1b7b6221977af6b89cae787b941&name=67c0b8c8fb29&r=67c0b8c8fb29b1b7b6221977af6b89cae787b941&format=side&path=/src/cmd/8g/cgen.c

R=rsc, remyoudompheng, r
CC=golang-dev
https://golang.org/cl/6450163