Fix clippy::needless_borrow in the compiler

`x clippy compiler -Aclippy::all -Wclippy::needless_borrow --fix`.

Then I had to remove a few unnecessary parens and muts that were exposed
now.

compiler/rustc_mir_transform/src/sroa.rs

@@ -167,7 +167,7 @@ impl<'tcx> ReplacementMap<'tcx> {
         };
         let fields = self.fragments[place.local].as_ref()?;
         let (_, new_local) = fields[f]?;
-        Some(Place { local: new_local, projection: tcx.mk_place_elems(&rest) })
+        Some(Place { local: new_local, projection: tcx.mk_place_elems(rest) })
     }
 
     fn place_fragments(