i965/fs: Handle non-const sample number in interpolateAtSample

i965/fs: Handle non-const sample number in interpolateAtSample

If a non-const sample number is given to interpolateAtSample it will
now generate an indirect send message with the sample ID similar to
how non-const sampler array indexing works. Previously non-const
values were ignored and instead it ended up using a constant 0 value.

The generator will try to determine if the sample ID is dynamically
uniform via nir_src_is_dynamically_uniform. If not it will query the
pixel interpolator in a loop, once for each different live sample
number. The next live sample number is found using emit_uniformize. If
multiple live channels have the same sample number then they will be
handled in a single iteration of the loop. The loop is necessary
because the indirect send message doesn't seem to have a way to
specify a different value for each fragment.

This fixes the following two Piglit tests:

arb_gpu_shader5-interpolateAtSample-nonconst
arb_gpu_shader5-interpolateAtSample-dynamically-nonuniform

v2: Handle dynamically non-uniform sample ids.
v3: Remove the BREAK instruction and predicate the WHILE directly.
    Make the tokens arrays const. (Matt Turner)
v4: Iterate over the live channels instead of each possible sample
    number.
v5: Don't special case immediate values in
    brw_pixel_interpolator_query. Make a better wrapper for the
    function to set up the PI send instruction. Ensure that the SHL
    instructions are scalar. (Francisco Jerez).

Reviewed-by: Francisco Jerez <currojerez@riseup.net>