[PatternMatch, InstSimplify] enhance m_AllOnes() to ignore undef elements in vectors

Loosening the matcher definition reveals a subtle bug in InstSimplify (we should not
assume that because an operand constant matches that it's safe to return it as a result).

So I'm making that change here too (that diff could be independent, but I'm not sure how 
to reveal it before the matcher change).

This also seems like a good reason to *not* include matchers that capture the value.
We don't want to encourage the potential misstep of propagating undef values when it's
not allowed/intended.

I didn't include the capture variant option here or in the related rL325437 (m_One), 
but it already exists for other constant matchers.

llvm-svn: 325466