cmd/compile: combine some extensions with loads

cmd/compile: combine some extensions with loads

For cases where we already have the ops, combine
sign or zero extension with the previous load
(even if the load is larger width).

Update #15105

Change-Id: I76c5ddd69e1f900d2a17d35503083bd3b4978e48
Reviewed-on: https://go-review.googlesource.com/28190
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>