[release-branch.go1.12] cmd/compile: guard against loads with negative offset...

[release-branch.go1.12] cmd/compile: guard against loads with negative offset from readonly constants

CL 154057 adds guards agaist out-of-bound reads from readonly
constants. It turns out that in dead code, the offset can also
be negative. Guard against negative offset as well.

Fixes #30257.

Change-Id: I47c2a2e434dd466c08ae6f50f213999a358c796e
Reviewed-on: https://go-review.googlesource.com/c/162819


Reviewed-by: Keith Randall <khr@golang.org>
(cherry picked from commit dca707b2)
Reviewed-on: https://go-review.googlesource.com/c/162827


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>