Reland "[SimplifyCFG] `FoldBranchToCommonDest()`: deal with mismatched IV's in...

Reland "[SimplifyCFG] `FoldBranchToCommonDest()`: deal with mismatched IV's in PHI's in common successor block"

This reverts commit 37b8f09a,
and returns commit 1bd0b82e.
The miscompile was in InstCombine, and it has been addressed.

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

Differential Revision: https://reviews.llvm.org/D139275