Replace some unnecessary vector copies with references.

llvm-svn: 190770
diff --git a/llvm/lib/Target/SystemZ/SystemZConstantPoolValue.cpp b/llvm/lib/Target/SystemZ/SystemZConstantPoolValue.cpp
index e9c4f6d..6c70811 100644
--- a/llvm/lib/Target/SystemZ/SystemZConstantPoolValue.cpp
+++ b/llvm/lib/Target/SystemZ/SystemZConstantPoolValue.cpp
@@ -39,7 +39,7 @@
 int SystemZConstantPoolValue::
 getExistingMachineCPValue(MachineConstantPool *CP, unsigned Alignment) {
   unsigned AlignMask = Alignment - 1;
-  const std::vector<MachineConstantPoolEntry> Constants = CP->getConstants();
+  const std::vector<MachineConstantPoolEntry> &Constants = CP->getConstants();
   for (unsigned I = 0, E = Constants.size(); I != E; ++I) {
     if (Constants[I].isMachineConstantPoolEntry() &&
         (Constants[I].getAlignment() & AlignMask) == 0) {