cmd/compile: fix 4-byte unaligned load rules

cmd/compile: fix 4-byte unaligned load rules

The 2-byte rule was firing before the 4-byte rule, preventing
the 4-byte rule from firing.  Update the 4-byte rule to use
the results of the 2-byte rule instead.

Add some tests to make sure we don't regress again.

Fixes #17147

Change-Id: Icfeccd9f2b96450981086a52edd76afb3191410a
Reviewed-on: https://go-review.googlesource.com/29382
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>