Don't crash when the Assignments vector is empty.

Reported by Vincent Lejeune using an out-of-tree target.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@166398 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/RegisterCoalescer.cpp

@@ -1302,7 +1302,7 @@ public:
                    SmallVectorImpl<unsigned> &ShrinkRegs);
 
   /// Get the value assignments suitable for passing to LiveInterval::join.
-  const int *getAssignments() const { return &Assignments[0]; }
+  const int *getAssignments() const { return Assignments.data(); }
 };
 } // end anonymous namespace