[LoopVectorize] Add FastMathFlags to the select used for reductions with tail-folding

Based on reviewer comments on https://reviews.llvm.org/D126692 I've
added FastMathFlags to the select instruction used when tail-folding
with reductions. These flags can then be used by InstCombine to
decide upon the most optimal floating point identity value for
fadd/fsub. Doing so unlocks further optimisations, such as folding
selects into masked loads.

Differential Revision: https://reviews.llvm.org/D126778