[release-branch.go1.8] cmd/compile: remove unnecessary type conversions on s390x

[release-branch.go1.8] cmd/compile: remove unnecessary type conversions on s390x

Some rules insert MOVDreg ops to ensure that type changes are kept.
If there is no type change (or the input is constant) then the MOVDreg
can be omitted, allowing further optimization.

Reduces the size of the .text section in the asm tool by ~33KB.

For #19227.

Change-Id: I0f7b40d8dbcda73bca96eb6d2bf13f9ffa88f4b6
Reviewed-on: https://go-review.googlesource.com/37535
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>