[g-arc-opts] Enable the Global ARC Optimizer by default.

Keep in mind that there is still more work to be done in the optimizer related
to loops, partial merging, etc. But this is the most basic multiple basic block
optimizer that has the features we want.

Swift SVN r18707

lib/SILPasses/Passes.cpp

@@ -104,7 +104,7 @@ void swift::runSILOptimizationPasses(SILModule &Module,
     PM.add(createSimplifyCFG());
     PM.add(createDevirtualization());
     PM.add(createEnumSimplification());
-    PM.add(createARCOpts());
+    PM.add(createGlobalARCOpts());
     PM.add(createAllocBoxToStack());
     PM.add(createDeadObjectElimination());
     PM.add(createDCE());