[VectorCombine] Fix assert on compare extract index

[kerbowa]

Summary

Extract index could be a differnet integral type.

[lebedev.ri]

This is missing a test.

[kerbowa]

Add test.

[lebedev.ri]

LG
Visual inspection suggests we don't have such failure elsewhere in this pass.

llvm/lib/Transforms/Vectorize/VectorCombine.cpp

 /// Compare the relative costs of extracts followed by scalar operation vs.
 /// vector operation followed by extract:
 /// opcode (extelt V0, C), (extelt V1, C) --> extelt (opcode V0, V1), C
 /// Unless the vector op is much more expensive than the scalar op, this
 /// eliminates an extract.
 static bool isExtractExtractCheap(Instruction *Ext0, Instruction *Ext1,
                                   unsigned Opcode,
                                   const TargetTransformInfo &TTI) {
-  assert(Ext0->getOperand(1) == Ext1->getOperand(1) &&
-         isa<ConstantInt>(Ext0->getOperand(1)) &&
+  assert(isa<ConstantInt>(Ext0->getOperand(1)) &&
+         (cast<ConstantInt>(Ext0->getOperand(1))->getZExtValue() ==
+          cast<ConstantInt>(Ext1->getOperand(1))->getZExtValue()) &&
          "Expected same constant extract index");
 
   Type *ScalarTy = Ext0->getType();
   Type *VecTy = Ext0->getOperand(0)->getType();
   int ScalarOpCost, VectorOpCost;
 
   // Get cost estimates for scalar and vector versions of the operation.
   bool IsBinOp = Instruction::isBinaryOp(Opcode);