Update fmt and use of memcpy

I'm still not totally sure if this is the right way to implement the memcpy, but that portion
compiles correctly now. Now to fix the compile errors everywhere else :).

compiler/rustc_middle/src/mir/mod.rs

@@ -1668,7 +1668,7 @@ impl Debug for Statement<'_> {
                 ref src,
                 ref dst,
                 ref size,
-            }) => write!(fmt, "src {:?} -> dst {:?}, {:?} bytes", src, dst, size),
+            }) => write!(fmt, "copy_nonoverlapping(src={:?}, dst={:?},bytes={:?})", src, dst, size),
             Nop => write!(fmt, "nop"),
         }
     }