cmd/compile: memcombine if values being stored are from consecutive loads

cmd/compile: memcombine if values being stored are from consecutive loads

If we load 2 values and then store those 2 loaded values, we can likely
perform that operation with a single wider load and store.

Fixes #60709

Change-Id: Ifc5f92c2f1b174c6ed82a69070f16cec6853c770
Reviewed-on: https://go-review.googlesource.com/c/go/+/502295
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Keith Randall <khr@golang.org>