[release-branch.go1.9] cmd/compile: fix large load/store offsets on 386

Pointer arithemetic is done mod 2^32 on 386, so we can just
drop the high bits of any large constant offsets.

The bounds check will make sure wraparounds are never observed.

Fixes #21655

Change-Id: I68ae5bbea9f02c73968ea2b21ca017e5ecb89223
Reviewed-on: https://go-review.googlesource.com/82675


Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-on: https://go-review.googlesource.com/88324


Run-TryBot: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>