Subzero: Add a peephole to fuse cmpxchg w/ later cmp+branch.

crosstest/test_sync_atomic.h

 //===- subzero/crosstest/test_sync_atomic.h - Test prototypes ---*- C++ -*-===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This file declares the function prototypes for cross testing atomic
 // intrinsics.
 //
 //===----------------------------------------------------------------------===//
 
 #include "test_sync_atomic.def"
 
 #define X(inst, type)                                                   \
   type test_##inst(bool fetch_first, volatile type *ptr, type a);       \
   type test_alloca_##inst(bool fetch, volatile type *ptr, type a);      \
   type test_const_##inst(bool fetch, volatile type *ptr, type ignored);
 
 FOR_ALL_RMWOP_TYPES(X)
 #undef X
 
-#define X(type)   \
-  type test_val_cmp_swap(volatile type *ptr, type oldval, type newval);
+#define X(type)                                                                \
+  type test_val_cmp_swap(volatile type *ptr, type oldval, type newval);        \
+  type test_val_cmp_swap_loop(volatile type *ptr, type oldval, type newval);
 
 ATOMIC_TYPE_TABLE
 #undef X