Atomics: promote ARM's IR-based atomics pass to CodeGen.

Still only 32-bit ARM using it at this stage, but the promotion allows
direct testing via opt and is a reasonably self-contained patch on the
way to switching ARM64.

At this point, other targets should be able to make use of it without
too much difficulty if they want. (See ARM64 commit coming soon for an
example).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206485 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/CodeGen.cpp

@@ -20,6 +20,7 @@ using namespace llvm;
 
 /// initializeCodeGen - Initialize all passes linked into the CodeGen library.
 void llvm::initializeCodeGen(PassRegistry &Registry) {
+  initializeAtomicExpandLoadLinkedPass(Registry);
   initializeBasicTTIPass(Registry);
   initializeBranchFolderPassPass(Registry);
   initializeCodeGenPreparePass(Registry);