This adds to 'Bug:', 'Issue:' and 'Regres:' as accepted CL grep checks.

Fixes: b/143093187
Change-Id: Iae44d8e8e49f7534bb940ec977edb4afcddc308e
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/37449Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>