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

tools/opt/opt.cpp

@@ -351,8 +351,9 @@ int main(int argc, char **argv) {
   initializeInstrumentation(Registry);
   initializeTarget(Registry);
   // For codegen passes, only passes that do IR to IR transformation are
-  // supported. For now, just add CodeGenPrepare.
+  // supported.
   initializeCodeGenPreparePass(Registry);
+  initializeAtomicExpandLoadLinkedPass(Registry);
 
 #ifdef LINK_POLLY_INTO_TOOLS
   polly::initializePollyPasses(Registry);