Fix stack overflow issue in `redundant_pattern_matching`

Fixes rust-lang#7169

~~cc `@Jarcho` Since tomorrow is release day and we need to get this also fixed in beta, I'll just revert the PR instead of looking into the root issue. Your changes are good, so if you have an idea what could cause this stack overflow and know how to fix it, please open a PR that reverts this revert with a fix.~~

r? `@llogiq`

changelog: none (fixes stack overflow, but this was introduced in this release cycle)

Backport of rust-lang#7170 to beta

r? `@ghost`

changelog: none (fixes stack overflow, but this was introduced in this release cycle)

Now actually opened towards the `beta` branch.