Remove the last actual usage of RestrictionSet

Switch to checking BorrowKind values of loans instead of their
RestrictionSet values. This was the last code that made a decision
based on a RestrictionSet.

Author: Cameron Zwarich

src/librustc/middle/borrowck/check_loans.rs

@@ -294,25 +294,11 @@ impl<'a> CheckLoanCtxt<'a> {
                loan1.repr(self.tcx()),
                loan2.repr(self.tcx()));
 
-        // Restrictions that would cause the new loan to be illegal:
-        let illegal_if = match loan2.kind {
-            // Look for restrictions against mutation. These are
-            // generated by all other borrows.
-            ty::MutBorrow => RESTR_MUTATE,
-
-            // Look for restrictions against freezing (immutable borrows).
-            // These are generated by `&mut` borrows.
-            ty::ImmBorrow => RESTR_FREEZE,
-
-            // No matter how the data is borrowed (as `&`, as `&mut`,
-            // or as `&unique imm`) it will always generate a
-            // restriction against mutating the data. So look for those.
-            ty::UniqueImmBorrow => RESTR_MUTATE,
-        };
-        debug!("illegal_if={:?}", illegal_if);
+        if compatible_borrow_kinds(loan1.kind, loan2.kind) {
+            return true;
+        }
 
         for restr in loan1.restrictions.iter() {
-            if !restr.set.intersects(illegal_if) { continue; }
             if restr.loan_path != loan2.loan_path { continue; }
 
             let old_pronoun = if new_loan.loan_path == old_loan.loan_path {