cmd/compile: fix ANDI/SRWI merge on ppc64

The shift amount should be masked to avoid rotation values
beyond the numer of bits. In this case, if the shift amount
is 0, it should rotate 0, not 32.

Fixes #45589

Change-Id: I1e764497a39d0ec128e29af42352b70c70b2ecc5
Reviewed-on: https://go-review.googlesource.com/c/go/+/310569


Run-TryBot: Paul Murphy <murp@ibm.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Carlos Eduardo Seo <carlos.seo@linaro.org>
Trust: Carlos Eduardo Seo <carlos.seo@linaro.org>