Reassoc FMF should not optimize FMA(a, 0, b) to (b)

Optimizing (a * 0 + b) to (b) requires assuming that a is finite and not
NaN. DAGCombiner will do this optimization when the reassoc fast math
flag is set, which is not correct. Change DAGCombiner to only consider
UnsafeMath for this optimization.

Differential Revision: https://reviews.llvm.org/D130232



Co-authored-by: Andrea Faulds <andrea.faulds@arm.com>