Fix buildbot breakage

SP is only used in an assert. Caused by r316374.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316377 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index 41d352d..ef71318 100644
--- a/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -942,6 +942,7 @@
     assert(Regs.size() == RegVTs.size() && Regs.size() == ValueVTs.size() &&
            "No 1:1 mapping from clobbers to regs?");
     unsigned SP = TLI.getStackPointerRegisterToSaveRestore();
+    (void)SP;
     for (unsigned I = 0, E = ValueVTs.size(); I != E; ++I) {
       Ops.push_back(DAG.getRegister(Regs[I], RegVTs[I]));
       assert(