nv50/ir/util: fix BitSet issues

nv50/ir/util: fix BitSet issues

BitSet::allocate() is being used with the expectation that it would
leave the bitfield untouched if its size hasn't changed, however,
the function always zeroed the last word, which led to obscure bugs
with live set computation.

This also fixes BitSet::resize(), which was broken, but luckily not
being used.

Cc: "10.2 10.3" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>