Avoid redundant mutations before casts

[afd]

Sometimes the AST will feature a nest of cast expressions. E.g.:

int foo() {
  unsigned x;
  return static_cast<int>(x);
}

will yield an explicit cast that encloses two implicit casts.

It should suffice to mutate a cast expression only when:

• its sub-expression is not a cast expression
• it is an l-value to r-value cast