cmd/compile: redo flag constant ops for arm

Encode the flag results in an auxint field instead of having
one opcode per flag state. This helps us handle the new *noov
branches in a unified manner.

This is only for arm, arm64 is in a subsequent CL.

We could extend to other architectures as well, athough it would
only be cleanup, no behavioral change.

Update #39505

Change-Id: Ia46cea596faad540d1496c5915ab1274571543f0
Reviewed-on: https://go-review.googlesource.com/c/go/+/238077


Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>