cmd/compile: use 32-bit comparisons where possible on s390x

We use 32-bit operations for 8- and 16-bit arithmetic, so use them
for comparisons too. This won't change performance but it is more
consistent and makes testing 8- and 16-bit comparison codegen
slightly more straightforward (for follow up CL).

Also fix a typo and add some additional double sign and zero
extension rules to remove the operations inserted by the comparison
rules.

Change-Id: I89ec1b0e09cb8be8090cf007be283ad88bba75a4
Reviewed-on: https://go-review.googlesource.com/102556


Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>