[llvm] [VPlan] Introduce ExitPhi VPInstruction, use to create phi for FOR. (PR #94760)

[via llvm-commits]

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@@ -1191,6 +1200,10 @@ class VPInstruction : public VPRecipeWithIRFlags {
     SLPStore,
     ActiveLaneMask,
     ExplicitVectorLength,
+    /// Creates a scalar phi in a leaf VPBB with a single predecessor in VPlan.
+    /// The first operand is the incoming value from the predecessor in VPlan,
+    /// the second operand is the incoming value for all other predecessors.
+    ExitPhi,
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ayalz wrote:

Calling it `ExitPhi` may be confusing: this Phi resides in ScalarPH (or VectorEpilogueTripCheck?) rather than in `Exit` Block (which hosts Live Outs, conceptually).

How about `ResumePhi`, `InterimPhi`, `InterPhi` - given that it passes a value from one loop, and bypassings thereof, to a subsequent loop.

https://github.com/llvm/llvm-project/pull/94760