i965: Put CS local thread ID uniform in last push register

i965: Put CS local thread ID uniform in last push register

This thread ID uniform will be used to compute the
gl_LocalInvocationIndex and gl_LocalInvocationID values.

It is important for this uniform to be added in the last push constant
register. fs_visitor::assign_constant_locations is updated to make
sure this happens.

The reason this is important is that the cross-thread push constant
registers are loaded first, and the per-thread push constant registers
are loaded after that. (Broadwell adds another push constant upload
mechanism which reverses this order, but we are ignoring this for
now.)

v2:
 * Add variable in intrinsics lowering pass
 * Make sure the ID is pushed last in assign_constant_locations, and
   that we save a spot for the ID in the push constants

v3:
 * Simplify code based with Jason's suggestions.

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>