cmd/compile: use CMPWU for 32-bit or smaller unsigned Geq on ppc64{,le}

Fixes #18808.

Change-Id: I49b266380b9d6804c9f6563ebac9c7c0e05f37f6
Reviewed-on: https://go-review.googlesource.com/35890


Run-TryBot: Michael Munday <munday@ca.ibm.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>