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_middle/src/mir/statement.rs

@@ -159,7 +159,7 @@ impl<'tcx> Place<'tcx> {
 
     #[inline]
     pub fn as_ref(&self) -> PlaceRef<'tcx> {
-        PlaceRef { local: self.local, projection: &self.projection }
+        PlaceRef { local: self.local, projection: self.projection }
     }
 
     /// Iterate over the projections in evaluation order, i.e., the first element is the base with