cmd/compile/internal/ssa: improve masking codegen on PPC64

cmd/compile/internal/ssa: improve masking codegen on PPC64

Generate RLDIC[LR] instead of MOVD mask, Rx; AND Rx, Ry, Rz.
This helps reduce code size, and reduces the latency caused
by the constant load.

Similarly, for smaller-than-register values, truncate constants
which exceed the range of the value's type to avoid needing to
load a constant.

Change-Id: I6019684795eb8962d4fd6d9585d08b17c15e7d64
Reviewed-on: https://go-review.googlesource.com/c/go/+/515576
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>