refactor: casts: remove inner casts only if all signednesses match

this fixes the i32 -> u32 -> isize test case

Author: fw-immunant

c2rust-refactor/src/transform/casts.rs

@@ -166,7 +166,7 @@ fn check_double_cast<'tcx>(e_ty: SimpleTy, t1_ty: SimpleTy, t2_ty: SimpleTy) ->
         | (SameWidth, Extend(s))
         | (SameWidth, FromPointer(s))
         | (SameWidth, ToPointer(s))
-            if s == e_ty.is_signed() =>
+            if s == e_ty.is_signed() && s == t1_ty.is_signed() =>
         {
             DoubleCastAction::RemoveInner
         }