Restrict the cases where ptr_eq triggers

[samueltardieu]

ptr_eq was recently enhanced to lint on more cases of raw pointers comparison:

• lint on all raw pointer comparison, by proposing to use [core|std]::ptr::eq(lhs, rhs) instead of lhs == rhs;
• removing one symetric as usize on each size if needed
• peeling any level of as *[const|mut] _ if the remaining expression can still be coerced into the original one (i.e., is a ref or raw pointer to the same type as before)

The current change restricts the lint to the cases where at least one level of symetric as usize, or any conversion to a raw pointer, could be removed. For example, a direct comparaison of two raw pointers will not trigger the lint anymore.

changelog: [ptr_eq]: do not lint when comparing two raw pointers directly with no casts involved

Fixes #14525