cmd/compile/ssa: fix (MOVWZreg (RLWINM)) folding on PPC64
RLIWNM does not clear the upper 32 bits of the target register if
the mask wraps around (e.g 0xF000000F). Don't elide MOVWZreg for
such masks. All other usage clears the upper 32 bits.
Fixes #67844.
Change-Id: I11b89f1da9ae077624369bfe2bf25e9b7c9b79bc
Reviewed-on: https://go-review.googlesource.com/c/go/+/590896 Reviewed-by: Keith Randall <khr@golang.org> Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>