Fix a crasher trying to fold each element in a comparison between two vectors

Fix a crasher trying to fold each element in a comparison between two vectors
if one of the vectors didn't have elements (such as undef). Fixes PR 6096.

Fix an issue in the constant folder where fcmp (<2 x %ty>, <2 x %ty>) would
have <2 x i1> type if constant folding was successful and i1 type if it wasn't.
This exposed a related issue in the bitcode reader.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@94069 91177308-0d34-0410-b5e6-96231b3b80d8