cmd/compile: optimize loads from readonly globals into constants

Instead of
   MOVB go.string."foo"(SB), AX
do
   MOVB $102, AX

When we know the global we're loading from is readonly, we can
do that read at compile time.

I've made this arch-dependent mostly because the cases where this
happens often are memory->memory moves, and those don't get
decomposed until lowering.

Did amd64/386/arm/arm64. Other architectures could follow.

Update #26498

Change-Id: I41b1dc831b2cd0a52dac9b97f4f4457888a46389
Reviewed-on: https://go-review.googlesource.com/c/141118


Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>