[LegalizeDAG] Add asserts to verify the types of custom legalized operation matches the original node.

[craig.topper]

Summary

We've messed this up a few times recently on RISCV. Experiments
with these asserts found a couple issues on other targets as well.
They've all been cleaned up now so we can put in these asserts to
catch future issues

I had to waive Glue because ADDC/ADDE/etc legalization replaces
Glue with i32 on at least AArch64. X86 used to do the same before
we switched to ADDCARRY. So I guess that's just how that works.

[RKSimon]

LGTM - no objections - maybe add comments explaining why MVT::Glue is special cased?

llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp

case TargetLowering::Custom:
       LLVM_DEBUG(dbgs() << "Trying custom legalization\n");
       // FIXME: The handling for custom lowering with multiple results is
       // a complete mess.
       if (SDValue Res = TLI.LowerOperation(SDValue(Node, 0), DAG)) {
         if (!(Res.getNode() != Node || Res.getResNo() != 0))
           return;
 
         if (Node->getNumValues() == 1) {
+          // Verify the new types match the original. Glue is waived because
+          // ISD::ADDC can be legalized by replacing Glue with an integer type.
+          assert((Res.getValueType() == Node->getValueType(0) ||
+                  Node->getValueType(0) == MVT::Glue) &&
+                 "Type mismatch for custom legalized operation");
           LLVM_DEBUG(dbgs() << "Successfully custom legalized node\n");
           // We can just directly replace this node with the lowered value.
           ReplaceNode(SDValue(Node, 0), Res);
           return;
         }
 
         SmallVector<SDValue, 8> ResultVals;
-        for (unsigned i = 0, e = Node->getNumValues(); i != e; ++i)
+        for (unsigned i = 0, e = Node->getNumValues(); i != e; ++i) {
+          // Verify the new types match the original. Glue is waived because
+          // ISD::ADDC can be legalized by replacing Glue with an integer type.
+          assert((Res->getValueType(i) == Node->getValueType(i) ||
+                  Node->getValueType(i) == MVT::Glue) &&
+                 "Type mismatch for custom legalized operation");
           ResultVals.push_back(Res.getValue(i));
+        }
         LLVM_DEBUG(dbgs() << "Successfully custom legalized node\n");
         ReplaceNode(Node, ResultVals.data());
         return;
       }
       LLVM_DEBUG(dbgs() << "Could not custom legalize node\n");
       LLVM_FALLTHROUGH;
     case TargetLowering::Expand:
       if (ExpandNode(Node))