Auto merge of #116199 - Urgau:simplify-invalid_ref_casting, r=cjgillot

Simplify some of the logic in the `invalid_reference_casting` lint

This PR simplifies 2 areas of the logic for the `invalid_reference_casting` lint:
 - The init detection: we now use the newly added `expr_or_init` function instead of a manual detection
 - The ref-to-mut-ptr casting detection logic: I simplified this logic by caring less hardly about the order of the casting operations

Those two simplifications permits us to detect more cases, as can be seen in the test output changes.

Author: bors