[SimplifyCFG] FoldBranchToCommonDest(): don't deal with unconditional branches

[SimplifyCFG] FoldBranchToCommonDest(): don't deal with unconditional branches

The case where BB ends with an unconditional branch,
and has a single predecessor w/ conditional branch
to BB and a single successor of BB is exactly the pattern
SpeculativelyExecuteBB() transform deals with.
(and in this case they both allow speculating only a single instruction)

Well, or FoldTwoEntryPHINode(), if the final block
has only those two predecessors.

Here, in FoldBranchToCommonDest(), only a weird subset of that
transform is supported, and it's glued on the side in a weird way.
  In particular, it took me a bit to understand that the Cond
isn't actually a branch condition in that case, but just the value
we allow to speculate (otherwise it reads as a miscompile to me).
  Additionally, this only supports for the speculated instruction
to be an ICmp.

So let's just unclutter FoldBranchToCommonDest(), and leave
this transform up to SpeculativelyExecuteBB(). As far as i can tell,
this shouldn't really impact optimization potential, but if it does,
improving SpeculativelyExecuteBB() will be more beneficial anyways.

Notably, this only affects a single test,
but EarlyCSE should have run beforehand in the pipeline,
and then FoldTwoEntryPHINode() would have caught it.

This reverts commit rL158392 / commit d33f4efbfdef6ffccf212ab3e40a7673589085fd.