[llvm] [SelectionDAG] WidenVecOp_INSERT_SUBVECTOR - Replace `INSERT_SUBVECTOR` with series of `INSERT_VECTOR_ELT` (PR #124420)

[Simon Pilgrim via llvm-commits]

================
@@ -7070,8 +7073,23 @@ SDValue DAGTypeLegalizer::WidenVecOp_INSERT_SUBVECTOR(SDNode *N) {
     return DAG.getNode(ISD::INSERT_SUBVECTOR, SDLoc(N), VT, InVec, SubVec,
                        N->getOperand(2));
 
-  report_fatal_error("Don't know how to widen the operands for "
-                     "INSERT_SUBVECTOR");
+  // If the operands can't be widened legally, just replace the INSERT_SUBVECTOR
+  // with a series of INSERT_VECTOR_ELT
+  unsigned Idx = N->getConstantOperandVal(2);
----------------
RKSimon wrote:

Should we just check if IndicesValid ? Or just early bail out if !IndicesValid above?
```
if (!IndicesValid)
  report_fatal_error("Don't know how to widen the operands for INSERT_SUBVECTOR");
```

https://github.com/llvm/llvm-project/pull/124420