Reland [SimplifyCFG] FoldBranchToCommonDest: lift use-restriction on bonus instructions
Original commit message:
- It doesn't make sense to enforce that the bonus instruction is only used once in it's basic block. What matters is whether those user instructions fit within our budget, sure, but that is another question.
- It doesn't make sense to enforce that said bonus instructions are only used within their basic block. Perhaps the branch condition isn't using the value computed by said bonus instruction, and said bonus instruction is simply being calculated to be used in successors?
So iff we can clone bonus instructions, to lift these restrictions,
we just need to carefully update their external uses
to use the new cloned instructions.
Notably, this transform (even without this change) appears to be
poison-unsafe as per alive2, but is otherwise (including the patch) legal.
We don't introduce any new PHI nodes, but only "move" the instructions
around, i'm not really seeing much potential for extra cost modelling
for the transform, especially since now we allow at most one such
bonus instruction by default.
This causes the fold to fire +11.4% more (13216 -> 14725)
as of vanilla llvm test-suite + RawSpeed.
The motivational pattern is IEEE-754-2008 Binary16->Binary32
^ that should be a switch, but it is not now: https://godbolt.org/z/bvja5v
That being said, even thought this seemed like this would fix it: https://godbolt.org/z/xGq3TM
apparently that fold is happening somewhere else afterall,
so something else also has a similar 'artificial' restriction.