SI-7126 Account for the alias types that don't dealias.

After this change:

    qbin/scalac -Ydebug test/files/pos/t7126.scala 2>&1 | grep warning
    warning: dropExistential did not progress dealiasing Test.this.T[Test.this.T], see SI-7126
    one warning found

`T[T]`? Really? The true bug lies somewhere else; the comments of
the ticket illuminate the general areas of concern.

This is a shallow fix intended for 2.10.1. I'll keep investigating the deeper issue, to help inform the decision about whether we are comfortable releasing 2.10.1 with only this bandaid.

Review by @paulp