i965/vs: Add a unit test for opt_compute_to_mrf().

i965/vs: Add a unit test for opt_compute_to_mrf().

The compute-to-mrf code is really twitchy, and it's hard to construct
GLSL testcases for it.  This unit test is also really hard to work with
(for example, if your instruction is removed by dead code elimination,
you end up inspecting something irrelevant), but I did use it for
debugging some of the commits to follow.

I called it test_vec4_register_coalesce because the compute-to-mrf code
is about to morph into that.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>