[llvm] [VPlan] Simplify pow-of-2 (mul|udiv) -> (shl|lshr) (PR #172477)

[Luke Lau via llvm-commits]

================
@@ -1345,6 +1345,21 @@ static void simplifyRecipe(VPSingleDefRecipe *Def, VPTypeAnalysis &TypeInfo) {
     return Def->replaceAllUsesWith(
         Def->getOperand(0) == A ? Def->getOperand(1) : Def->getOperand(0));
 
+  const APInt *APC;
+  if (match(Def, m_c_Mul(m_VPValue(), m_APInt(APC))) && APC->isPowerOf2())
+    return Def->replaceAllUsesWith(Builder.createNaryOp(
+        Instruction::Shl,
+        {Def->getOperand(0),
+         Plan->getConstantInt(APC->getBitWidth(), APC->exactLogBase2())},
+        *cast<VPRecipeWithIRFlags>(Def), Def->getDebugLoc()));
+
+  if (match(Def, m_UDiv(m_VPValue(), m_APInt(APC))) && APC->isPowerOf2())
+    return Def->replaceAllUsesWith(Builder.createNaryOp(
+        Instruction::LShr,
+        {Def->getOperand(0),
+         Plan->getConstantInt(APC->getBitWidth(), APC->exactLogBase2())},
+        *cast<VPRecipeWithIRFlags>(Def), Def->getDebugLoc()));
----------------
lukel97 wrote:

I don't think udiv flags are compatible with lshr, we can probably omit the copy?

```suggestion
        {}, Def->getDebugLoc()));
```

https://github.com/llvm/llvm-project/pull/172477