[InstSimplify] fold select of vector constants that include undef elements

As mentioned in D72643, we'd like to be able to assert that any select
of equivalent constants has been removed before we're deep into InstCombine.

But there's a loophole in that assertion for vectors with undef elements
that don't match exactly.

This patch should close that gap. If we have undefs, we can't safely
propagate those unless both constants elements for that lane are undef.

Differential Revision: https://reviews.llvm.org/D72958