[llvm] [Analysis] Optimise mul(const, (udiv %n, const)) during SCEV expansion (PR #212769)

[Paul Walker via llvm-commits]

================
@@ -599,6 +599,26 @@ Value *SCEVExpander::visitAddExpr(SCEVUseT<const SCEVAddExpr *> S) {
 Value *SCEVExpander::visitMulExpr(SCEVUseT<const SCEVMulExpr *> S) {
   Type *Ty = S->getType();
 
+  // Specializations for 2-operand cases.
+  const SCEVConstant *MulC;
+  const SCEV *Op1;
+  if (match(S, m_scev_Mul(m_SCEVConstant(MulC), m_SCEV(Op1)))) {
+    // mul(PowerOf2C, (udiv X, PowerOf2C)) == (X >> C) << C
+    //  -> X & (-1 << C)
+    const SCEV *Val;
+    if (MulC->getAPInt().isPowerOf2() &&
+        match(Op1, m_scev_UDiv(m_SCEV(Val), m_scev_Specific(MulC)))) {
----------------
paulwalker-arm wrote:

I was thinking something more like
```
 if (match(S, m_scev_Mul(m_SCEVConstant(MulC),
                         m_scev_UDiv(m_SCEV(Val), m_scev_Specific(MulC)))) &&
     MulC->getAPInt().isPowerOf2()) {
```
?

https://github.com/llvm/llvm-project/pull/212769