[ConstantFolding] Handle bitcasts of undef fp vector elements

We used the wrong type for constructing a zero vector element which led
to type mismatches.

This fixes PR28771.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@277197 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed