code refactor, modify compile-fail tests

src/test/compile-fail/borrowck/borrowck-issue-14498.rs

@@ -27,7 +27,7 @@ fn indirect_write_to_imm_box() {
     let y: Box<_> = box &mut x;
     let p = &y;
     ***p = 2; //[ast]~ ERROR cannot assign to data in a `&` reference
-              //[mir]~^ ERROR cannot assign to immutable item `***p`
+              //[mir]~^ ERROR cannot assign through `&`-reference `p`
     drop(p);
 }
 

src/test/compile-fail/borrowck/borrowck-overloaded-index-ref-index.rs

@@ -70,5 +70,5 @@ fn main() {
     };
     s[2] = 20;
     //[ast]~^ ERROR cannot assign to immutable indexed content
-    //[mir]~^^ ERROR cannot assign to immutable item
+    //[mir]~^^ ERROR cannot assign through immutable item
 }