[x86] Make computing the zeroable elements slightly more powerful, at

least in theory.

I don't actually have a test case that benefits from this, but
theoretically, it could come up, and I don't want to try to think about
whether this is the culprit or something else is, so I'd rather just
make this code powerful. =/ Makes me sad that I can't really test it
though.

llvm-svn: 229298