[ConstantFold] fp_binop undef, undef --> undef

These are uncontroversial and independent of a proposed LangRef edits (D44216).

I tried to fix tests that would fold away:
rL327004
rL327028
rL327030
rL327034

I'm not sure if the Reassociate tests are meaningless yet, but they probably will be 
as we add more folds, so if anyone has suggestions or wants to fix those, please do.

Differential Revision: https://reviews.llvm.org/D44258

llvm-svn: 327058