[llvm] [RISCV] Move strength reduction of mul X, 3/5/9*2^N to combine (PR #89966)

[Yingwei Zheng via llvm-commits]

================
@@ -13426,10 +13426,27 @@ static SDValue expandMul(SDNode *N, SelectionDAG &DAG,
     if (MulAmt % Divisor != 0)
       continue;
     uint64_t MulAmt2 = MulAmt / Divisor;
-    // 3/5/9 * 2^N -> shXadd (sll X, C), (sll X, C)
-    // Matched in tablegen, avoid perturbing patterns.
-    if (isPowerOf2_64(MulAmt2))
-      return SDValue();
+    // 3/5/9 * 2^N ->  shl (shXadd X, X), N
+    if (isPowerOf2_64(MulAmt2)) {
+      SDLoc DL(N);
+      SDValue X = N->getOperand(0);
+      // Put the shift first if we can fold a zext into the
+      // shift forming a sll.uw.
----------------
dtcxzyw wrote:

```suggestion
      // shift forming a slli.uw.
```

https://github.com/llvm/llvm-project/pull/89966