mesa: Reduce memory usage for reg alloc with many graph nodes (part 2).

mesa: Reduce memory usage for reg alloc with many graph nodes (part 2).

After the previous fix that almost removes an allocation of 4*n^2
bytes, we can use a bitset to reduce another allocation from n^2 bytes
to n^2/8 bytes.

Between the previous commit and this one, the peak heap size for an
oglconform ARB_fragment_program max instructions test on i965 goes from
4GB to 255MB.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=55825
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>