i965/fs: Bake regs_written into the IR instead of recomputing it later.

i965/fs: Bake regs_written into the IR instead of recomputing it later.

For sampler messages, it depends on the target gen, and on gen4
SIMD16-sampler-on-SIMD8-execution we were returning 4 instead of 8 like we
should.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
NOTE: This is a candidate for the 9.1 branch.