cmd/compile: refactor memory op constructors in SSA builder

Pulling these operations out separately so it's easier to add
instrumentation calls.

Passes toolstash-check.

Updates #19054.

Change-Id: If6a537124a87bac2eceff1d66d9df5ebb3bf07be
Reviewed-on: https://go-review.googlesource.com/102816Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>