Get the in-scope set right during RULE matching

There was a subtle error in the in-scope set during RULE matching,
which led to #20200 (not the original report, but the reports of
failures following an initial bug-fix commit).

This patch fixes the problem, and simplifies the code a bit.
In pariticular there was a very mysterious and ad-hoc in-scope set
extension in rnMatchBndr2, which is now moved to the right place,
namely in the Let case of match, where we do the floating.

I don't have a small repro case, alas.

(cherry picked from commit 4ff4d434)