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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229298 91177308-0d34-0410-b5e6-96231b3b80d8