SI-7612 Fix SOE regression in lubs

The enclosed test case (whittled down lovingly from lift-mapper)
triggers the cycle in `elimSub`.

Regressed in a06d31f6a. The change was dicussed
<https://github.com/scala/scala/pull/2022#discussion_r2832997>.

This patch reverts to using `.underlying`.

Below, I've shown the different paths that are taken through
that method with before and after this patch. The key divergence
is when `<refinement>.this.M.dealias` reveals a captured existential
type symbol.

A more detailed look at how that happens:
   <https://gist.github.com/retronym/5874775>

Review by @adriaanm.

Targetting 2.10.x on grounds of locality of the fix and regression from 2.10.0 -> 2.10.1.