cmd/compile/internal/ssa: on PPC64, try combining CLRLSLDI and SRDconst into RLWINM

cmd/compile/internal/ssa: on PPC64, try combining CLRLSLDI and SRDconst into RLWINM

This provides a small performance bump to crc64 as measured on ppc64le/power10:

name              old time/op    new time/op    delta
Crc64/ISO64KB       49.6µs ± 0%    46.6µs ± 0%  -6.18%
Crc64/ISO4KB        3.16µs ± 0%    2.97µs ± 0%  -5.83%
Crc64/ISO1KB         840ns ± 0%     794ns ± 0%  -5.46%
Crc64/ECMA64KB      49.6µs ± 0%    46.5µs ± 0%  -6.20%
Crc64/Random64KB    53.1µs ± 0%    49.9µs ± 0%  -6.04%
Crc64/Random16KB    15.9µs ± 1%    15.0µs ± 0%  -5.73%

Change-Id: I302b5431c7dc46dfd2d211545c483bdcdfe011f1
Cq-Include-Trybots: luci.golang.try:gotip-linux-ppc64_power10,gotip-linux-ppc64_power8,gotip-linux-ppc64le_power8,gotip-linux-ppc64le_power9,gotip-linux-ppc64le_power10
Reviewed-on: https://go-review.googlesource.com/c/go/+/581937
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Eli Bendersky <eliben@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>