There are multiple LLVM passes that very much move the
intrinsic using the descriptor outside of the loop, defeating
the entire point of creating the loop.

Defeat the optimizer by  splitting the break into a separate
if-statement and putting an optimization barrier on the bool
in between.

v2: Move from a callback based system to begin/end loop.
    This does not make it significantly less intrusive but
    is a bit nicer with all the extra struct and callback
    stubs.
v3: Deal with non-divergent values in divergent path.

Closes: mesa/mesa#2160
Fixes: 028ce527

 "radv: Add non-uniform indexing lowering."
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Tested-by: Marge Bot <mesa/mesa!4109>
Part-of: <mesa/mesa!4109>