cmd/compile: combine similar code in amd64's assembly generator

This CL combines similar code in amd64's assembly generator. The
total size of pkg/linux_amd64/cmd/compile/ decreases about 4.5KB,
while the generated amd64 code is not affected.

Change-Id: I4cdbdd22bde8857aafdc29b47fa100a906fa1598
Reviewed-on: https://go-review.googlesource.com/c/140298


Run-TryBot: Ben Shi <powerman1st@163.com>
Reviewed-by: Keith Randall <khr@golang.org>