cmd/compile/internal/ssa: remove useless zero extension

We generate MOVBLZX for byte-sized LoadReg, so
(MOVBQZX (LoadReg (Arg))) is the same as
(LoadReg (Arg)). Remove those zero extension where possible.
Triggers several times during all.bash.

Fixes #25378
Updates #15300

Change-Id: If50656e66f217832a13ee8f49c47997f4fcc093a
Reviewed-on: https://go-review.googlesource.com/115617


Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>