[instsimplify] consistently handle undef and out of bound indices for...

[instsimplify] consistently handle undef and out of bound indices for insertelement and extractelement

In one case, we were handling out of bounds, but not undef indices.  In the other, we were handling undef (with the comment making the analogy to out of bounds), but not out of bounds.  Be consistent and treat both undef and constant out of bounds indices as producing undefined results.

As a side effect, this also protects instcombine from having to handle large constant indices as we always simplify first.

llvm-svn: 321575