[ConstProp] allow folding for fma that produces NaN

[ConstProp] allow folding for fma that produces NaN

Folding for fma/fmuladd was added here:
rL202914
...and as seen in existing/unchanged tests, that works to propagate NaN
if it's already an input, but we should fold an fma() that creates NaN too.

From IEEE-754-2008 7.2 "Invalid Operation", there are 2 clauses that apply
to fma, so I added tests for those patterns:

  c) fusedMultiplyAdd: fusedMultiplyAdd(0, ∞, c) or fusedMultiplyAdd(∞, 0, c)
     unless c is a quiet NaN; if c is a quiet NaN then it is implementation
     defined whether the invalid operation exception is signaled
  d) addition or subtraction or fusedMultiplyAdd: magnitude subtraction of
     infinities, such as: addition(+∞, −∞)

Differential Revision: https://reviews.llvm.org/D67446

llvm-svn: 371735