Auto merge of #149096 - chenyukang:yukang-fix-unused-vars-148373, r=oli-obk

Skip unused variables warning for unreachable code

Fixes rust-lang/rust#148373

These warnings are not reported on stable branch, but are now reported on the beta.

I tried another solution to record whether a `local` is reachable in `find_dead_assignments`, but the code in this PR seems simpler.

r? `@cjgillot`

Author: bors