[X86] Call removeDeadNode when we're done doing custom isel for mul, div and test

Summary:
Once we've done our custom isel for these nodes, I think we should be calling removeDeadNode to prune them out of the DAG. Table driven isel ultimately either calls morphNodeTo which modifies a node and doesn't leave dead nodes. Or it emits new nodes and then calls removeDeadNode as part of Opc_CompleteMatch.

If you run a simple multiply test case like this through llc with -debug you'll see a umul_lohi node get printed as part of the dump for Instruction Selection ends.

```
define i64 @foo(i64 %a, i64 %b) local_unnamed_addr #0 {
entry:
  %conv = zext i64 %a to i128
  %conv1 = zext i64 %b to i128
  %mul = mul nuw nsw i128 %conv1, %conv
  %shr = lshr i128 %mul, 64
  %conv2 = trunc i128 %shr to i64
  ret i64 %conv2
}
```

Reviewers: RKSimon, spatel, zvi, guyblank, niravd

Reviewed By: niravd

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D37547

llvm-svn: 312857

llvm/lib/Target/X86/X86ISelDAGToDAG.cpp

@@ -2612,6 +2612,7 @@ void X86DAGToDAGISel::Select(SDNode *Node) {
       DEBUG(dbgs() << "=> "; ResHi.getNode()->dump(CurDAG); dbgs() << '\n');
     }
 
+    CurDAG->RemoveDeadNode(Node);
     return;
   }
 
@@ -2796,6 +2797,7 @@ void X86DAGToDAGISel::Select(SDNode *Node) {
       ReplaceUses(SDValue(Node, 1), Result);
       DEBUG(dbgs() << "=> "; Result.getNode()->dump(CurDAG); dbgs() << '\n');
     }
+    CurDAG->RemoveDeadNode(Node);
     return;
   }
 
@@ -2854,6 +2856,7 @@ void X86DAGToDAGISel::Select(SDNode *Node) {
         // one, do not call ReplaceAllUsesWith.
         ReplaceUses(SDValue(Node, (Opcode == X86ISD::SUB ? 1 : 0)),
                     SDValue(NewNode, 0));
+        CurDAG->RemoveDeadNode(Node);
         return;
       }
 
@@ -2889,6 +2892,7 @@ void X86DAGToDAGISel::Select(SDNode *Node) {
         // one, do not call ReplaceAllUsesWith.
         ReplaceUses(SDValue(Node, (Opcode == X86ISD::SUB ? 1 : 0)),
                     SDValue(NewNode, 0));
+        CurDAG->RemoveDeadNode(Node);
         return;
       }
 
@@ -2909,6 +2913,7 @@ void X86DAGToDAGISel::Select(SDNode *Node) {
         // one, do not call ReplaceAllUsesWith.
         ReplaceUses(SDValue(Node, (Opcode == X86ISD::SUB ? 1 : 0)),
                     SDValue(NewNode, 0));
+        CurDAG->RemoveDeadNode(Node);
         return;
       }
 
@@ -2929,6 +2934,7 @@ void X86DAGToDAGISel::Select(SDNode *Node) {
         // one, do not call ReplaceAllUsesWith.
         ReplaceUses(SDValue(Node, (Opcode == X86ISD::SUB ? 1 : 0)),
                     SDValue(NewNode, 0));
+        CurDAG->RemoveDeadNode(Node);
         return;
       }
     }