cmd/compile: fix s390x load-combining rules

MOVD{reg,nop} operations (added in CL 36256) inserted to preserve
type information were blocking the load-combining rules. Fix this
by merging type changes into loads wherever possible.

Fixes #19059.

Change-Id: I8a1df06eb0f231b40ae43107d4a3bd0b9c441b59
Reviewed-on: https://go-review.googlesource.com/36843


Run-TryBot: Michael Munday <munday@ca.ibm.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>