SI-8478 Fix a performance regression in subtyping

When checking `M[X] <:< M[Y]` for an `M` with an invariant
parameter, we have to check that `X <:< Y && Y <:< X`. This is
done in `isSubArgs`.

The compile time of that program in the ticket jumps from 20s in
2.10.4 to too-long-to-measure in 2.11.0. This commit reverts the
a subtle change to `isSubArgs` in ea93654 that was ultimately
responsible.

The search for this was unusually circuitious, even for scalac.
It appeared in 9c09c1709 due a tiny error that has since been
reverted in 58bfa19. But 58bfa19 still exhibited abysmal performance,
due to an intervening regression that I'm targeting here.

I haven't managed to extract a performance test from Slick. Using
the test that @cvogt provided, however, with this patch:

	% time qbin/scalac -J-Xmx4G -classpath /Users/jason/code/slick-presentation/target/scala-2.10/classes:/Users/jason/.sbt/0.13/staging/b64b71d1228cdfe7b6d8/slick/target/scala-2.10/classes:/Users/jason/.ivy2/cache/org.slf4j/slf4j-api/jars/slf4j-api-1.6.4.jar /Users/jason/code/slick-presentation/src/main/scala/SlickPresentation.scala

	real	0m21.853s
	user	0m33.625s
	sys	0m0.878s

Which is back to 2.10.x style performance.
Merge pull request #3694 from retronym/ticket/8478-2

SI-8478 Fix a performance regression in subtyping