[llvm] [BDCE] Handle multi-use `select` of `and`s on demanded bits (PR #79688)

[Yingwei Zheng via llvm-commits]

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@@ -125,6 +125,33 @@ static bool bitTrackingDCE(Function &F, DemandedBits &DB) {
       }
     }
 
+    // Simplify select of ands when the mask does not affect the demanded bits.
+    if (SelectInst *SI = dyn_cast<SelectInst>(&I)) {
+      APInt Demanded = DB.getDemandedBits(SI);
+      if (!Demanded.isAllOnes()) {
+        for (Use &U : I.operands()) {
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dtcxzyw wrote:

```suggestion
        for (Use &U : I.operands().drop_front()) {
```


https://github.com/llvm/llvm-project/pull/79688