nv50/ir: fix bb positions after exit instructions

nv50/ir: fix bb positions after exit instructions

It's fairly rare that the BB layout puts BBs after the exit block, which
is likely the reason these issues lingered for so long.

This fixes a fraction of issues with the giant pixmark piano shader.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Cc: <mesa-stable@lists.freedesktop.org>
(cherry picked from commit e988999791c32fd2594986fde742367d7f71c724)