Some original shift opcodes for ppc64x expected an operand to be
a mask instead of a shift count, preventing some valid shift counts
from being written.

This adds new opcodes for shifts where needed, using mnemonics that
match the ppc64 asm and allowing the assembler to accept the full set
of valid shift counts.

Fixes #15016

Change-Id: Id573489f852038d06def279c13fd0523736878a7
Reviewed-on: https://go-review.googlesource.com/31853
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Carlos Eduardo Seo <cseo@linux.vnet.ibm.com>
Reviewed-by: David Chase <drchase@google.com>