cmd/compile: restrict ppc64 constant shifts to amount 0-63

cmd/compile: restrict ppc64 constant shifts to amount 0-63

... and 0-31 for 32-bit shifts.

Generally update the docs for ppc64 shift instructions to be
clearer about what they actually do.

This issue is causing problems for the subsequent CL. The shift
amount was <0 and caused the assembler to report an invalid instruction.

Change-Id: I8c708a15e7f71931835e6e543d8db3c716186e52
Reviewed-on: https://go-review.googlesource.com/c/go/+/232858
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>