This tries to approach the problem noted by @arsenm:
terrible codegen for __builtin_fpclassify():
https://godbolt.org/z/388zqdE37
Just because the PHI in the common successor happens to have different
incoming values for these two blocks, doesn't mean we have to give up.
It's quite easy to deal with this, we just need to produce a select:
https://alive2.llvm.org/ce/z/000srb
Now, the cost model for this transform is rather overly strict,
so this will basically never fire. We tally all (over all preds)
the selects needed to the NumBonusInsts
Recepie?