cmd/compile: Combine smaller loads into a larger load

cmd/compile: Combine smaller loads into a larger load

This only deals with the loads themselves.  The bounds checks
are a separate issue.  Also doesn't handle stores, those are
harder because we need to make sure intermediate memory states
aren't observed (which is hard to do with rewrite rules).

Use one byte shorter instructions for zero-extending loads.

Update #14267

Change-Id: I40af25ab5208488151ba7db32bf96081878fa7d9
Reviewed-on: https://go-review.googlesource.com/20218
Reviewed-by: Alexandru Moșoi <alexandru@mosoi.ro>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>