[InstSimplify] fold fma/fmuladd with a NaN or undef operand

This is intended to be similar to the constant folding results from
D67446
and earlier, but not all operands are constant in these tests, so the
responsibility for folding is left to InstSimplify.

Differential Revision: https://reviews.llvm.org/D67721

llvm-svn: 373455