cmd/compile/internal/ssa: refactor zeroUpper32Bits

cmd/compile/internal/ssa: refactor zeroUpper32Bits

Explicitly whitelist args of OpSelect{1|2} that zero upper 32 bits.
Use better values in corresponding test.
This should have been a part of  CL 96815, but it was submitted, before
relevant comments.

Change-Id: Ic85d90a4471a17f6d64f8f5c405f21378bf3a30d
Reviewed-on: https://go-review.googlesource.com/97295
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>