glsl: Use foreach_list in lower_jumps.cpp

glsl: Use foreach_list in lower_jumps.cpp

The visitor class in lower_jumps.cpp never removes or replaces the
instruction being visited, but it frequently alters or removes the
instructions that follow it.  Therefore, to make sure the altered IR
is visited, it needs to iterate through exec_lists using foreach_list
rather than visit_exec_list().

Without this patch, lower_jumps.cpp may require multiple passes in
order to lower all jumps.  This results in sub-optimal output because
lower_jumps.cpp produces a brand new set of temporary variables each
time it is run, and the redundant temporary variables are not
guaranteed to be eliminated by later optimization passes.

Also, certain invariants assumed by lower_jumps.cpp may fail to hold,
causing assertion failures.

Fixes unit tests test_lower_pulled_out_jump,
test_lower_unified_returns, test_lower_guarded_conditional_break,
test_lower_return_non_void_at_end_of_loop, and test_lower_returns_3.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>