i965/fs: Try to emit LINE instructions on Gen <= 5.

i965/fs: Try to emit LINE instructions on Gen <= 5.

The LINE instruction performs a multiply-add instruction (a * b + c)
where b and c are scalar arguments. It reads b and c from offsets in
src0 such that you can load them (it they're representable) as a
vector-float immediate with a single instruction.

Hurts some programs, but that'll all get better once we CSE the
vector-float MOVs in the next patch.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=77544
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>