If a phi has fast math flags, we can propagate it to the widened select.
To do this, this patch makes VPPhi and VPBlendRecipe subclasses of
VPRecipeWithIRFlags, and propagates it through PlainCFGBuilder and
VPPredicator.
Alive2 proofs for some of the FMFs (it looks like it can't reason about
the full "fast" set yet)
nnan: https://alive2.llvm.org/ce/z/f0bRd4
nsz: https://alive2.llvm.org/ce/z/u9P96T
The actual motivation for this to eventually be able to move the special
casing for tail folding in
LoopVectorizationPlanner::addReductionResultComputation into the CFG in
#176143, which requires passing through FMFs.
8cd86ff284