[ConstantFold] fp_binop AnyConstant, undef --> NaN

With the updated LangRef ( D44216 / rL327138 ) in place, we can proceed with more constant folding.

I'm intentionally taking the conservative path here: no matter what the constant or the FMF, we can 
always fold to NaN. This is because the undef operand can be chosen as NaN, and in our simplified 
default FP env, nothing else happens - NaN just propagates to the result. If we find some way/need 
to propagate undef instead, that can be added subsequently.

The tests show that we always choose the same quiet NaN constant (0x7FF8000000000000 in IR text). 
There were suggestions to improve that with a 'NaN' string token or not always print a 64-bit hex 
value, but those are independent changes. We might also consider setting/propagating the payload of 
NaN constants as an enhancement.

Differential Revision: https://reviews.llvm.org/D44308

llvm-svn: 327208