[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 dca707b2a040642bb46aa4da4fb4eb6188cc2502)
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>
2 files changed
tree: 1d8d72d8c43947be97781686ee057831e5e9c3ea
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  7. LICENSE
  8. PATENTS
  9. README.md
  10. VERSION
  11. api/
  12. doc/
  13. favicon.ico
  14. lib/
  15. misc/
  16. robots.txt
  17. src/
  18. test/
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