i965: Add nir based intrinsic lowering and thread ID uniform

i965: Add nir based intrinsic lowering and thread ID uniform

We add a lowering pass for nir intrinsics. This pass can replace nir
intrinsics with driver specific nir lower code.

We lower the gl_LocalInvocationIndex intrinsic based on a uniform
which is loaded with a thread specific ID.

We also lower the gl_LocalInvocationID based on
gl_LocalInvocationIndex.

v2:
 * Create variable during lowering pass. (Ken)

v3:
 * Don't create a variable, but instead just insert an intrisic call
   to load a uniform from the allocated location. (Jason)

v4:
 * Don't run this pass if thread_local_id_index < 0

Cc: "12.0" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>