cmd/compile: use a boolean as a avoid clobbering flags mov marker

cmd/compile: use a boolean as a avoid clobbering flags mov marker

The Value type implements Aux interface because it is being used as a
"avoid clobbering flags" marker by amd64, x86 and s390x SSA parts.

Create a boolean that implements the Aux interface. Use it as the marker
instead. We no longer need Value to implement Aux.

Resolves a TODO.

See CL 275756 for more info.

Change-Id: I8a1eddf7e738b8aa31e82f3c4c590bafd2cdc56b
Reviewed-on: https://go-review.googlesource.com/c/go/+/461156
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Jakub Ciolek <jakub@ciolek.dev>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>