[instsimplify] Move (extelt (inselt Vec, Value, Index), Index) -> Value from InstCombine

[dsanders]

Summary

As requested in https://reviews.llvm.org/D135625#3858141

[nikic]

LGTM, thanks.

llvm/lib/Analysis/InstructionSimplify.cpp

if (isa<FixedVectorType>(VecVTy) && IdxC->getValue().uge(MinNumElts))
       return PoisonValue::get(VecVTy->getElementType());
     // Handle case where an element is extracted from a splat.
     if (IdxC->getValue().ult(MinNumElts))
       if (auto *Splat = getSplatValue(Vec))
         return Splat;
     if (Value *Elt = findScalarElement(Vec, IdxC->getZExtValue()))
       return Elt;
   } else {
+    // extractelt x, (insertelt y, elt, n), n -> elt
+    // If the possibly-variable indices are trivially known to be equal
+    // (because they are the same operand) then use the value that was
+    // inserted directly.
+    auto *IE = dyn_cast<InsertElementInst>(Vec);
+    if (IE && IE->getOperand(2) == Idx)
+      return IE->getOperand(1);
+
     // The index is not relevant if our vector is a splat.
     if (Value *Splat = getSplatValue(Vec))
       return Splat;
   }
   return nullptr;
 }
 
 Value *llvm::simplifyExtractElementInst(Value *Vec, Value *Idx,

llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp

if (match(SrcVec, m_Cmp(Pred, m_Value(X), m_Value(Y))) &&
     // extelt (cmp X, Y), Index --> cmp (extelt X, Index), (extelt Y, Index)
     Value *E0 = Builder.CreateExtractElement(X, Index);
     Value *E1 = Builder.CreateExtractElement(Y, Index);
     return CmpInst::Create(cast<CmpInst>(SrcVec)->getOpcode(), Pred, E0, E1);
   }
 
   if (auto *I = dyn_cast<Instruction>(SrcVec)) {
     if (auto *IE = dyn_cast<InsertElementInst>(I)) {
-      // If the possibly-variable indices are trivially known to be equal
-      // (because they are the same operand) then use the value that was
-      // inserted directly.
-      if (IE->getOperand(2) == Index)
-        return replaceInstUsesWith(EI, IE->getOperand(1));
       // instsimplify already handled the case where the indices are constants
       // and equal by value, if both are constants, they must not be the same
       // value, extract from the pre-inserted value instead.
       if (isa<Constant>(IE->getOperand(2)) && IndexC)
         return replaceOperand(EI, 0, IE->getOperand(0));
     } else if (auto *GEP = dyn_cast<GetElementPtrInst>(I)) {
       auto *VecType = cast<VectorType>(GEP->getType());
       ElementCount EC = VecType->getElementCount();

llvm/test/Transforms/InstSimplify/vector-inseltextelt.ll

 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt < %s -passes=instcombine -S | FileCheck %s
+; RUN: opt < %s -passes=instsimplify -S | FileCheck %s
 
 ; extracting a just-inserted element should yield the original value
 define float @constant_index(<4 x float> %x, float %val) {
 ; CHECK-LABEL: @constant_index(
 ; CHECK-NEXT:    ret float [[VAL:%.*]]
 ;
   %ins = insertelement <4 x float> %x, float %val, i32 1
   %ext = extractelement <4 x float> %ins, i32 1