cmd/compile: remove more extension ops when not needed

cmd/compile: remove more extension ops when not needed

If we're not using the upper bits, don't bother issuing a
sign/zero extension operation.

For arm64, after CL 520916 which fixed a correctness bug with
extensions but as a side effect leaves many unnecessary ones
still in place.

Change-Id: I5f4fe4efbf2e9f80969ab5b9a6122fb812dc2ec0
Reviewed-on: https://go-review.googlesource.com/c/go/+/521496
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>