Currently the shift amount is only masked on x86. Change it so it
is masked on all architectures. In the worst case we generate a
couple of extra instructions to perform the masking and in the best
case we can elide overflow checks.

This particular shift could also be replaced with a rotate
instruction during optimization which would remove both the masking
instructions and overflow checks on all architectures.

Fixes #31165.

Change-Id: I16b7a8800b4ba8813dc83735dfc59564e661d3b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/170122
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>