Don't record chain or glue operands in stackmap DAG nodes.

In the last LLVM sync, I tickled a bug in the "large stackmap locations"
support that we added to ykllvm a while back. It manifested when building
yklua. An "I shouldn't have already visited this node" assertion fails:

```
ld.lld: llvm/lib/CodeGen/SelectionDAG/ScheduleDAGSDNodes.cpp:393: void llvm::ScheduleDAGSDNodes::BuildSchedUnits(): Assertion `N->getNodeId() == -1 && "Node already inserted!"' failed.
```

Upon inspecting the DAGs, the stackmap nodes contain glue and chain operands as
constituents of live variables. That makes zero sense! We are effectively
asking the compiler to record the location of something which isn't a piece of
data!

It causes two incoming glue and chains edges for some stackmap nodes, hence the
assertion failure (two visits instead of one).

This change omits chain and glue nodes when pushing stackmap operands and adds
an assertion to catch this problem should it arise again.

Author: vext01

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -9531,6 +9531,11 @@ void SelectionDAGBuilder::populateCallLoweringInfo(
           Call->countOperandBundlesOfType(LLVMContext::OB_preallocated) != 0);
 }
 
+// Is the node `N` a glue or chain node?
+bool isGlueOrChain(const SDValue &N) {
+  return (N.getValueType() == MVT::Other) || (N.getValueType() == MVT::Glue);
+}
+
 /// Given the stackmap live variable `N`, search its sub-DAG and return all of
 /// the constituent values that need to be reported in the stackmap table.
 static std::vector<SDValue> findLiveConstituents(SelectionDAG &DAG,
@@ -9542,8 +9547,11 @@ static std::vector<SDValue> findLiveConstituents(SelectionDAG &DAG,
   case ISD::CONCAT_VECTORS:
   case ISD::MERGE_VALUES:
   case ISD::BUILD_VECTOR:
-    for (SDValue Op : N->op_values())
-      V.push_back(Op);
+    for (SDValue Op : N->op_values()) {
+      if (!isGlueOrChain(Op)) {
+        V.push_back(Op);
+      }
+    }
     break;
   case ISD::INSERT_VECTOR_ELT: {
     V = findLiveConstituents(DAG, N.getOperand(0));
@@ -9556,6 +9564,14 @@ static std::vector<SDValue> findLiveConstituents(SelectionDAG &DAG,
     V.push_back(N);
   }
 
+  // We have to be careful not to ask the stackmap to record the locations
+  // of glue or chain operands. That would make no sense, as glue and chain
+  // operands are not real data, rather control dependencies that are an
+  // artefact of using a DAG.
+  for (SDValue &VV: V) {
+    assert(!isGlueOrChain(VV));
+  }
+
   return V;
 }