v3d/compiler: allow to batch spills

v3d/compiler: allow to batch spills

Some shaders that need to spill hundreds of registers can take very long times
to compile as each allocation attempt spills a single register and restarts
the allocation process. We can significantly cut down these times if we allow
the compiler to spill in batches, which should be possible if we are spilling
uniforms, which is in fact the kind of spills that we do first because they
have lower cost than TMU spills.

Doing this could cause us to slightly over spill in some cases (depending on
the chosen batch size) leading to slightly worse performance, so we only
enable this behavior after we have started to spill over a certain threshold,
at which point we assume that performance won't be good and we want to
favor compilation speed instead.

v2:
  - Keep it simple and just try to spill a fixed amount of registers in a
    batch instead of trying to compute this dynamically based on accumulated
    spills and current register pressure. (Eric).

v3:
  - Check if the node is valid before doing anything with it.
  - Drop the environment variable to select batch size and just fix it to 20.

With this we can take this CTS test from 35 minutes down to about 3 minutes:
dEQP-VK.ssbo.layout.random.all_shared_buffer.5

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6766>