[llvm] [VPlan] Remove MaskedCond and preserve SSA in early exits (PR #201784)

[Andrei Elovikov via llvm-commits]

================
@@ -3235,20 +3224,19 @@ bool VPlanTransforms::handleUncountableEarlyExits(
               m_BranchOnCond(m_VPValue(CondOfEarlyExitingVPBB)));
     assert(Matched && "Terminator must be BranchOnCond");
 
-    // Insert the MaskedCond in the EarlyExitingVPBB so the predicator adds
-    // the correct block mask.
     VPBuilder EarlyExitingBuilder(EarlyExitingVPBB->getTerminator());
-    auto *CondToEarlyExit = EarlyExitingBuilder.createNaryOp(
-        VPInstruction::MaskedCond,
+    auto *CondToEarlyExit =
         TrueSucc == ExitBlock
             ? CondOfEarlyExitingVPBB
-            : EarlyExitingBuilder.createNot(CondOfEarlyExitingVPBB));
-    assert((isa<VPIRValue>(CondOfEarlyExitingVPBB) ||
-            !VPDT.properlyDominates(EarlyExitingVPBB, LatchVPBB) ||
-            VPDT.properlyDominates(
-                CondOfEarlyExitingVPBB->getDefiningRecipe()->getParent(),
-                LatchVPBB)) &&
-           "exit condition must dominate the latch");
+            : EarlyExitingBuilder.createNot(CondOfEarlyExitingVPBB);
+
+    // Add phis so there's a def of CondToEarlyExit on every path leading to the
+    // latch. The condition is false on paths that didn't go through
+    // EarlyExitingVPBB.
+    DenseMap<VPBasicBlock *, VPValue *> Defs = {{HeaderVPBB, Plan.getFalse()}};
+    Defs[EarlyExitingVPBB] = CondToEarlyExit;
+    CondToEarlyExit = vputils::reconstructSSA(LatchVPBB, Defs);
----------------
eas wrote:

Nit:
```suggestion
    CondToEarlyExit = vputils::reconstructSSA(LatchVPBB,
                                              {{HeaderVPBB, Plan.getFalse()}, 
                                               {EarlyExitingVPBB, CondToEarlyExit});
```
likewise below.

https://github.com/llvm/llvm-project/pull/201784