Auto merge of #128550 - compiler-errors:shadowed-params-perf, r=petro…

…chenkov

Only walk ribs to collect possibly shadowed params if we are adding params in our new rib

No need to collect params from parent ribs if we literally have no params to declare in this new rib.

Attempt to win back some of the perf in rust-lang/rust#128357 (comment).

Please review with whitespace *off*, the diff should be like 2 lines.

r? petrochenkov