r600g/sb: fix scheduling of PRED_SET instructions

r600g/sb: fix scheduling of PRED_SET instructions

PRED_SET instructions that update exec mask should be scheduled immediately
prior to the "if-then-else" block, because any instruction that is
inserted after alu clause with PRED_SET and before conditional block is
also conditionally executed by hw (exec mask is already updated at that
moment).

Propbably it's better to make PRED_SET a part of conditional
"if-then-else" block in the IR to handle this more cleanly,
but for now this temporary solution should prevent the problem.

Signed-off-by: Vadim Girlin <vadimgirlin@gmail.com>