i965/fs_nir: Handle sample ID, position, and mask better

i965/fs_nir: Handle sample ID, position, and mask better

Before, we were emitting the full pile of setup instructions for sample_id
and sample_pos every time they were used.  With this commit, we emit them
in their own pass once at the beginning of the shader and simply emit uses
later on.  When it comes time for setting up VS, we can put setup for its
special values in the same pass.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>