cmd/compile: remove unnecessary type conversions on s390x

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.

Change-Id: I386883bb35b843c7b99a269cd6840dca77cf4371
Reviewed-on: https://go-review.googlesource.com/36547
Run-TryBot: Michael Munday <munday@ca.ibm.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>