[llvm] [VPlan] Model branch cond to enter scalar epilogue in VPlan. (PR #92651)

[Florian Hahn via llvm-commits]

fhahn wrote:

> Nice step forward expanding VPlan's scope! Preliminary comments before diving in:
> 
> Commit message needs updating.
> 

Thanks, should be updated now!

> In general, VPlan backpatches branches when reaching their destination VPBB::execute() and VPBB::createEmptyBasicBlock(). This includes backpatching the branch leaving the vector loop when reaching its destination middle/exit VPBB. Could the branch ending middle block be backpatched similarly, when reaching its destination VPBB's - during VPIRBB::execute()?

This is what the patch currently should do, the branch from the vector region's exiting block to the middle block is patched in `createEmptyBasicBlock` when the middle block is visited. Branch from the middle block is updated in VPIRbasicBlock::execute, together with the dominator tree.

https://github.com/llvm/llvm-project/pull/92651