Optimize the sequence:
	MOV $imm, Rt
	ADD Rt, Rs, Rd

into:
	ADD $imm, Rs, Rd

Saves 66k in godoc.

Change-Id: I27b4aaa0ec80a59472fe2e5816efdf3db9c901ee
Reviewed-on: https://go-review.googlesource.com/8632Reviewed-by: Minux Ma <minux@golang.org>