Fix stable reference check for And types with type params

[HarrisL2]

Fixes existing bug that also resurfaced in open community build with explicit-nulls enabled in the following project
neandertech/langoustine

How much have you relied on LLM-based tools in this contribution?

Extensively, for producing the appropriate fix

How was the solution tested?

New automated tests at
tests/explicit-nulls/pos/opt.scala
and a previously disabled test at
tests/explicit-nulls/pos/flow-stable.scala

[noti0na1]

I remember a long time (at least 5yr) ago @sjrd remind me something about stable checking on andtypes is not trival.

[HarrisL2]

I see, this was mostly an LLM generated fix and I'm not familiar with the nuance of this section of code.