[release-branch.go1.8] cmd/compile: fix subword store/load elision for amd64, x86, arm

Replacing byteload-of-bytestore-of-x with x is incorrect
when x contains a larger-than-byte value (and so on for
16 and 32-bit load/store pairs).  Replace "x" with the
appropriate zero/sign extension of x, which if unnecessary
will be repaired by other rules.

Made logic for arm match x86 and amd64; yields minor extra
optimization, plus I am (much) more confident it's correct,
despite inability to reproduce bug on arm.

Ppc64 lacks this optimization, hence lacks this problem.

See related https://golang.org/cl/37154/
Fixes #20530.

[Merge conflicts in generated rewrite files resolved by
regenerating from scratch, using the programs in ssa/gen.]

Change-Id: I6af9cac2ad43bee99cafdcb04725ce7e55a43323
Reviewed-on: https://go-review.googlesource.com/44355


Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-on: https://go-review.googlesource.com/70841


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>