[llvm] [InstCombine] Fold `X udiv Y` to `X lshr cttz(Y)` if Y is a power of 2 (PR #121386)

[via llvm-commits]

================
@@ -1623,14 +1623,27 @@ Instruction *InstCombinerImpl::visitUDiv(BinaryOperator &I) {
     return Lshr;
   }
 
-  // Op1 udiv Op2 -> Op1 lshr log2(Op2), if log2() folds away.
-  if (takeLog2(Builder, Op1, /*Depth*/ 0, /*AssumeNonZero*/ true,
-               /*DoFold*/ false)) {
-    Value *Res = takeLog2(Builder, Op1, /*Depth*/ 0,
-                          /*AssumeNonZero*/ true, /*DoFold*/ true);
+  auto GetShiftableDenom = [&](Value *Denom) -> Value * {
+    // Op0 udiv Op1 -> Op0 lshr log2(Op1), if log2() folds away.
+    if (takeLog2(Builder, Denom, /*Depth=*/0, /*AssumeNonZero=*/true,
+                 /*DoFold=*/false))
+      return takeLog2(Builder, Denom, /*Depth=*/0, /*AssumeNonZero=*/true,
+                      /*DoFold=*/true);
----------------
veera-sivarajan wrote:

Thanks for the feedback.

But that causes `llvm/test/Transforms/InstCombine/div-shift.ll` to fail.
```
LLVM ERROR: Instruction Combining on udiv_umin_ did not reach a fixpoint after 1 iterations.
```

https://github.com/llvm/llvm-project/pull/121386