ir3: Fix bug with shaders that only exit via discard

ir3: Fix bug with shaders that only exit via discard

discard is supposed to be a terminator, killing the thread, so that it's
possible to exit main solely by a discard e.g. inside of an infinite
loop. However, it currently isn't treated as a terminator in NIR due to
workarounds turning it into demote (d3d-style kill) and even if that
were fixed, we probably wouldn't want to treat discard_if as a jump
since otherwise the scheduler wouldn't be able to schedule things around
it. So, add this workaround which inserts jump instructions as
necessary to guarantee that the program always terminates.

This fixes a hang in dEQP-VK.graphicsfuzz.while-inside-switch, which
conditionally does a discard inside an infinite loop.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4658>