ir3/ra: Fix available bitset for live-through collect srcs

ir3/ra: Fix available bitset for live-through collect srcs

When we mark live-through sources that are merged with the destination
as killed, we kept the bitsets in sync, but we forgot to keep them in
sync when unmarking them after allocating the destination. The result
was that "available" wasn't correct for any instruction afterwards. This
resulted in a bad register allocation with IR3_SHADER_DEBUG=spillall for
a dEQP-VK test.

While we're changing this, use ra_foreach_src().

Fixes: 0ffcb19b9d9 ("ir3: Rewrite register allocation")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12033>