i965: Enable the new FS backend on pre-gen6 as well.

i965: Enable the new FS backend on pre-gen6 as well.

It is now to the point where we have no regressing piglit tests.  It
also fixes Yo Frankie! and Humus DynamicBranching, probably due to the
piglit bias tests that work that didn't on the Mesa IR backend.

As a downside, performance takes about a 5-10% performance hit at the
moment (e.g. nexuiz 19.8fps -> 18.8fps), which I plan to resolve by
reintroducing 16-wide fragment shaders where possible.  It is a win,
though, for fragment shaders using flow control.