[ConstantFolding] Fold bitcasts of vectors w/ undef elements

An undef vector element can be treated as if it had any value.  Folding
such a vector element to 0 in a bitcast can open up further folding
opportunities.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@277104 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed