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_borrowck/src/used_muts.rs

@@ -35,7 +35,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 never_initialized_mut_locals: &mut never_initialized_mut_locals,
                 mbcx: self,
             };
-            visitor.visit_body(&visitor.mbcx.body);
+            visitor.visit_body(visitor.mbcx.body);
         }
 
         // Take the union of the existed `used_mut` set with those variables we've found were