i965/fs: use SSA values directly

i965/fs: use SSA values directly

Before, we would use registers, but set a magical "parent_instr" field
to indicate that it was actually purely an SSA value (i.e., it wasn't
involved in any phi nodes). Instead, just use SSA values directly, which
lets us get rid of the hack and reduces memory usage since we're not
allocating a nir_register for every value. It also makes our handling of
load_const more consistent compared to the other instructions.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>