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

BSFQ/BSRQ/BSFL/BSRL/SQRTSD have similar logic in amd64's assembly
generator. This CL combines them together while does not impact
generated amd64 code. The total size of
pkg/linux_amd64/cmd/compile/internal decreases about 1.8KB.

Change-Id: I5f3210c5178c20ac9108877c69f17234baf5b6b7
Reviewed-on: https://go-review.googlesource.com/c/140438


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