Stop putting operations after a tail call.

This prevents the emission of DAG-generated vreg definitions after a
tail call be dropping them entirely (on the grounds that nothing could
use them anyway, and they interfere with O0 CodeGen).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185754 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -5357,6 +5357,10 @@ void SelectionDAGBuilder::LowerCallTo(ImmutableCallSite CS, SDValue Callee,
     // As a special case, a null chain means that a tail call has been emitted and
     // the DAG root is already updated.
     HasTailCall = true;
+
+    // Since there's no actual continuation from this block, nothing can be
+    // relying on us setting vregs for them.
+    PendingExports.clear();
   } else {
     DAG.setRoot(Result.second);
   }