Fix if_let_mutex not checking Mutexes behind refs
Fixes#9193
We can always peel references because we are looking for a method-call, for which autoderef applies.
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changelog: [`if_let_mutex`]: detect calls to `Mutex::lock()` if mutex is behind a ref
changelog: [`if_let_mutex`]: Add labels to the two instances of the same Mutex that will deadlock
0fc95e88f5