i965/fs: Improve CSE performance by expiring some available expressions.

i965/fs: Improve CSE performance by expiring some available expressions.

We're already walking the list, and we can easily know when something
has no reason to be in the list any longer, so take a brief extra step
to reduce our worst-case runtime (an oglconform test that emits the
maximum instructions in a fragment program).  I don't actually know what
the worst-case runtime was, because it was too long and I got bored.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>