cmd/compile: PPC64, elide unnecessary sign extension

cmd/compile: PPC64, elide unnecessary sign extension

Inputs to store[BHW] and cmpW(U) need not be correct
in more bits than are used by the instruction.

Added a pattern tailored to what appears to be cgo boilerplate.
Added a pattern (also seen in cgo boilerplate and hashing)
to replace {EQ,NE}-CMP-ANDconst with {EQ-NE}-ANDCCconst.
Added a pattern to clean up ANDconst shift distance inputs
(this was seen in hashing).

Simplify repeated and,or,xor.

Fixes #17109.

Change-Id: I68eac83e3e614d69ffe473a08953048c8b066d88
Reviewed-on: https://go-review.googlesource.com/30455
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>