cmd/compile: repair mask optimizations

Had sense of the instructions wrong.
XORQ means a Q of mask bits (64),
not mask bits for Qs (2/4/8).

Includes a not-comprehensive-across operations
test that IS comprehensive across width x count
combinations, to slightly validate use of "B"
mask operations (8 bits) for results of 2 and 4
element comparisons.

Fixes #80140.

Change-Id: I7537bb3327ae124d216f24e4fa054510d8c7cf3c
Reviewed-on: https://go-review.googlesource.com/c/go/+/794680
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
3 files changed
tree: d7d507ebe50667ceacfc2846948c3c56dbdbecc0
  1. .github/
  2. api/
  3. doc/
  4. lib/
  5. misc/
  6. src/
  7. test/
  8. .gitattributes
  9. .gitignore
  10. codereview.cfg
  11. CONTRIBUTING.md
  12. go.env
  13. LICENSE
  14. PATENTS
  15. README.md
  16. SECURITY.md
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