llvm

4cdb0335 - [VPlan] Optimize pre-increment IV latch users with tail folding (#206499)

[VPlan] Optimize pre-increment IV latch users with tail folding (#206499) This was noticed after #204089 caused IndVarsSimplify to convert some live out IV users to use the pre-incremented IV, not the post-incremented. Tail folded live-outs don't have the `(extract-last-lane (extract-last-part foo))` form, but instead have the form `(extract-lane (last-active-lane header-mask), foo)`. For post-incremented IVs in tail folding, these are converted to VPInstruction::ExitingIVValue which are handled separately. But ExitingIVValue can't be used for the pre-incremented IV. So this teaches optimizeLatchExitInductionUser to detect the last-active-lane of the header mask form.