[SelectionDAG] unroll unsupported vector FP ops earlier to avoid libcalls on...

[SelectionDAG] unroll unsupported vector FP ops earlier to avoid libcalls on undef elements (PR38527)

This solves the motivating case from:
https://bugs.llvm.org/show_bug.cgi?id=38527

If we are legalizing an FP vector op that maps to 1 of the LLVM intrinsics that mimic libm calls, 
but we're going to end up with scalar libcalls for that vector type anyway, then we should unroll 
the vector op into scalars before widening. This avoids libcalls because we've lost the knowledge 
that some of the scalar elements are undef.

Differential Revision: https://reviews.llvm.org/D50791

llvm-svn: 340469