broadcom/vc5: Add support for register spilling.

broadcom/vc5: Add support for register spilling.

Our register spilling support is nice to have since vc4 couldn't at all,
but we're still very restricted due to needing to not spill during a TMU
operation, or during the last segment of the program (which would be nice
to spill a value of, when there's a long-lived value being passed through
with little modification from the start to the end).

We could do better by emitting unspills for the last-segment values just
before the last thrsw, since the last segment is probably not the maximum
interference area.

Fixes GTF uniform_buffer_object_arrays_of_all_valid_basic_types and 3
others.