nv50/ir: copy joinAt when splitting both before and after

nv50/ir: copy joinAt when splitting both before and after

The current implementation only moves the joinAt when splitting after
the given instruction, not before it. So if you have a BB with

  foo
  instr
  bar
  joinat

and thus with joinAt set, we end up first splitting before instr, at
which point the instr's bb is updated to the new bb. Since that bb
doesn't have a joinAt set (despite containing one), when splitting after
the instr, there is nothing to copy over. Since the joinat will be in
the "split" bb irrespective of whether we're splitting before or after
the instruction, move it over in either case.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91124
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "10.5 10.6" <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 5dcb28c3d26828ed1b0e2bd5a0589c5baab04b85)