[Transforms] Fix a cast to Instruction if CreateLShr() returns a constant

[slydiman]

Summary

When X is a constant, CreateLShr() returns a constant instead of an instruction.

[lebedev.ri]

Test missing, and i strongly suspect this is dead code, it should already happen automatically.

[nikic]

I've taken the liberty of deleting this code entirely in https://github.com/llvm/llvm-project/commit/efece08ae27d8ace684c5119d6696b1a18b2150f. We rely on automatic debug loc assignment everywhere else in InstCombine, I don't see any reason why this fold would need special treatement.

[slydiman]

Thanks. I will close this revision.

llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp

// FIXME: This should not be limited to scalar (pull into APInt match above).
     if (match(Op1, m_ConstantInt(C3)) &&
         match(Op0, m_LShr(m_Xor(m_Value(X), m_ConstantInt(C1)),
                           m_ConstantInt(C2))) &&
         Op0->hasOneUse()) {
       // fold (C1 >> C2) ^ C3
       APInt FoldConst = C1->getValue().lshr(C2->getValue());
       FoldConst ^= C3->getValue();
       // Prepare the two operands.
-      auto *Opnd0 = cast<Instruction>(Builder.CreateLShr(X, C2));
+      Value *Opnd0 = Builder.CreateLShr(X, C2);
       Opnd0->takeName(cast<Instruction>(Op0));
-      Opnd0->setDebugLoc(I.getDebugLoc());
+      // When X is a constant, CreateLShr() returns a constant instead
+      // of an instruction.
+      if (Instruction *Opnd0Inst = dyn_cast<Instruction>(Opnd0))
+        Opnd0Inst->setDebugLoc(I.getDebugLoc());
       return BinaryOperator::CreateXor(Opnd0, ConstantInt::get(Ty, FoldConst));
     }
   }
 
   if (Instruction *FoldedLogic = foldBinOpIntoSelectOrPhi(I))
     return FoldedLogic;
 
   // Y ^ (X | Y) --> X & ~Y