The CMPWUconst op (32-bit unsigned comparison with immediate) takes
an unsigned immediate value. In SSA this should be sign extended to
64-bits to match the Int32 type given in the op and then zero
extended when producing the final assembly. Before this CL we were
zero extending in SSA which caused ssacheck to fail.

While we are here also ensure other 32-bit immediates are sign
extended in SSA.

Passes toolstash -cmp on std on s390x.

Fixes #22611.

Change-Id: I5c061a76a710b10ecb0650c9c42efd9fa1c123cc
Reviewed-on: https://go-review.googlesource.com/76336
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>