[DAGCombiner] clean up extract-of-concat fold; NFC

This hopes to improve readability and adds an assert.
The functional change noted by the TODO comment is
proposed in:
D72361

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -18566,19 +18566,27 @@ SDValue DAGCombiner::visitEXTRACT_SUBVECTOR(SDNode *N) {
     }
   }
 
-  // Combine:
-  //    (extract_subvec (concat V1, V2, ...), i)
-  // Into:
-  //    Vi if possible
-  // Only operand 0 is checked as 'concat' assumes all inputs of the same
-  // type.
-  if (V.getOpcode() == ISD::CONCAT_VECTORS && isa<ConstantSDNode>(Index) &&
-      V.getOperand(0).getValueType() == NVT) {
-    unsigned Idx = N->getConstantOperandVal(1);
-    unsigned NumElems = NVT.getVectorNumElements();
-    assert((Idx % NumElems) == 0 &&
-           "IDX in concat is not a multiple of the result vector length.");
-    return V->getOperand(Idx / NumElems);
+  if (V.getOpcode() == ISD::CONCAT_VECTORS && isa<ConstantSDNode>(Index)) {
+    EVT ConcatSrcVT = V.getOperand(0).getValueType();
+    assert(ConcatSrcVT.getVectorElementType() == NVT.getVectorElementType() &&
+           "Concat and extract subvector do not change element type");
+
+    unsigned ExtIdx = N->getConstantOperandVal(1);
+    unsigned ExtNumElts = NVT.getVectorNumElements();
+    assert(ExtIdx % ExtNumElts == 0 &&
+           "Extract index is not a multiple of the input vector length.");
+
+    unsigned ConcatSrcNumElts = ConcatSrcVT.getVectorNumElements();
+    unsigned ConcatOpIdx = ExtIdx / ConcatSrcNumElts;
+
+    // If the concatenated source types match this extract, it's a direct
+    // simplification:
+    // extract_subvec (concat V1, V2, ...), i --> Vi
+    if (ConcatSrcNumElts == ExtNumElts)
+      return V.getOperand(ConcatOpIdx);
+
+    // TODO: Handle the case where the concat operands are larger than the
+    //       result of this extract by extracting directly from a concat op.
   }
 
   V = peekThroughBitcasts(V);