[ARM] Register the Thumb2SizeReducePass. NFC

Also adds a simple test case.

llvm-svn: 321072
diff --git a/llvm/lib/Target/ARM/ARM.h b/llvm/lib/Target/ARM/ARM.h
index 3aac689..9ffb4c2 100644
--- a/llvm/lib/Target/ARM/ARM.h
+++ b/llvm/lib/Target/ARM/ARM.h
@@ -61,6 +61,7 @@
 void initializeARMPreAllocLoadStoreOptPass(PassRegistry &);
 void initializeARMConstantIslandsPass(PassRegistry &);
 void initializeARMExpandPseudoPass(PassRegistry &);
+void initializeThumb2SizeReducePass(PassRegistry &);
 
 } // end namespace llvm