[llvm] InstCombine: Fix a crash in `PointerReplacer` when constructing a new PHI (PR #130256)

[Changpeng Fang via llvm-commits]

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@@ -360,7 +360,10 @@ void PointerReplacer::replace(Instruction *I) {
 
     IC.InsertNewInstWith(NewI, LT->getIterator());
     IC.replaceInstUsesWith(*LT, NewI);
-    WorkMap[LT] = NewI;
+    // LT has actually been replaced by NewI. It is useless to insert LT into
+    // the map. Instead, we insert NewI into the map to indicate this is the
+    // replacement (new value).
+    WorkMap[NewI] = NewI;
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changpeng wrote:

> Can you just not add it to the map? I'm not sure what the point of the identity mapping would be

The whole logic is that, when replacing a PHI instruction, all its operands are expected to have already been replaced and can be found in the map.  So getReplacement (Operand) should always return the replacement (never a NULL).
    Type *NewTy = getReplacement(PHI->getIncomingValue(0))->getType();

However, when a load is replaced, all the uses of the load has already been actually replaced by the new load:
IC.replaceInstUsesWith(*LT, NewI);

We will never search the map with the original load (LT). And we expect to find the replacement for the new load.
So we have to add the newI into the map instead.

https://github.com/llvm/llvm-project/pull/130256