redundant_locals: fix FPs on mutated shadows

Fixes #11290.

When a mutable binding is shadowed by
a mutable binding of the same name in a different scope, mutations in that scope have different meaning.
This PR fixes spurious `redundant_locals` emissions on such locals.

cc `@Centri3,` `@flip1995`

changelog: [`redundant_locals`]: fix false positives on mutated shadows