[llvm] [VPlan] Introduce scalar loop header in plan, remove VPLiveOut. (PR #109975)

[via llvm-commits]

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@@ -8876,24 +8898,8 @@ addUsersInExitBlock(VPlan &Plan,
 static void addLiveOutsForFirstOrderRecurrences(
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ayalz wrote:

"LiveOuts" in `addLiveOutsForFirstOrderRecurrences` should be replaced, say by renaming it `addExitUsersOfFixedOrderRecurrences`? It would handle only feeding Exit with penultimate value, if feeding the scalar loop with last value is handled by addScalarResumePhis().

All occurrences of "VPLiveOut" and "live-out" in documentation should be updated, as in "// Pass live-in values used by exit phis directly through to the live-out" above  - which is obsolete w/o having live outs.

https://github.com/llvm/llvm-project/pull/109975