cmd/compile/internal/ssa: fix ppc64 merging of (CLRLSLDI (SRD ...))

cmd/compile/internal/ssa: fix ppc64 merging of (CLRLSLDI (SRD ...))

The rotate value was not correctly converted from a 64 bit to 32
bit rotate. This caused a miscompile of
golang.org/x/text/unicode/runenames.Names.

Fixes #67526

Change-Id: Ief56fbab27ccc71cd4c01117909bfee7f60a2ea1
Reviewed-on: https://go-review.googlesource.com/c/go/+/586915
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>