[llvm] [InstCombine] Simplify nested selects with implied condition (PR #83739)

[via llvm-commits]

================
@@ -3867,5 +3867,22 @@ Instruction *InstCombinerImpl::visitSelectInst(SelectInst &SI) {
     }
   }
 
+  // Fold nested selects if the inner condition can be implied by the outer
+  // condition.
+  Value *InnerCondVal;
+  const DataLayout &DL = getDataLayout();
+  if (match(TrueVal,
+            m_Select(m_Value(InnerCondVal), m_Value(LHS), m_Value(RHS))) &&
+      CondVal->getType() == InnerCondVal->getType())
+    if (auto Implied =
+            isImpliedCondition(CondVal, InnerCondVal, DL, /*LHSIsTrue=*/true))
+      return replaceOperand(SI, 1, *Implied ? LHS : RHS);
+  if (match(FalseVal,
+            m_Select(m_Value(InnerCondVal), m_Value(LHS), m_Value(RHS))) &&
+      CondVal->getType() == InnerCondVal->getType())
+    if (auto Implied =
+            isImpliedCondition(CondVal, InnerCondVal, DL, /*LHSIsTrue=*/false))
+      return replaceOperand(SI, 2, *Implied ? LHS : RHS);
+
----------------
goldsteinn wrote:

Maybe put this in a lambda:
```
auto CanSimplfySelectArmAs = [...](bool Arm) {
 if(match(Arm ? TrueVal : FalseVal, m_Select(...)) && CondVal->getType() == ...)
  if(Implied = isImplied(..., Arm)
   return *Implied ? LHS : RHS;
}
if(auto * V = CanSimplfySelectArmAs(true)) replaceOperand(SI, 1, V);
if(auto * V = CanSimplfySelectArmAs(false)) replaceOperand(SI, 2, V);

```
?

There is a lot of duplication between the two arms.

https://github.com/llvm/llvm-project/pull/83739