lima/gpir: avoid invalid write in regalloc

lima/gpir: avoid invalid write in regalloc

Reported by gcc -fsanitize=address, sometimes gpir regalloc attempts to
handle an uninitialized node->value_reg (containing the value -1), which
results in an invalid array access.
Avoid it for now to prevent crashes, but more investigation may be
required later on.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Reviewed-by: Vasily Khoruzhick <anarsoul@gmail.com>
Cc: 22.0 <mesa-stable>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14894>