Only run the test on x86_64

Smaller platforms don't merge the loads the same way.

Author: scottmcm

src/test/codegen/swap-small-types.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 // compile-flags: -O
+// only-x86_64
 
 #![crate_type = "lib"]