cmd/compile: stop rematerializable ops from clobbering flags

cmd/compile: stop rematerializable ops from clobbering flags

Rematerializable ops can be inserted after the flagalloc phase,
they must therefore not clobber flags. This CL adds a check to
ensure this doesn't happen and fixes the instances where it
does currently.

amd64: ADDQconst and ADDLconst were recently changed to be
rematerializable in CL 54393 (only in tip, not 1.9). That change
has been reverted.

s390x: MOVDaddr could clobber flags when using dynamic linking due
to a ADD with immediate instruction. Change the code generation to
use LA/LAY instead.

Fixes #21080.

Change-Id: Ia85c882afa2a820a309e93775354b3169ec6d034
Reviewed-on: https://go-review.googlesource.com/63030
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Ilya Tocar <ilya.tocar@intel.com>