i965: Fix invalid pointer read in dead_control_flow_eliminate().

There may not be a previous block.  In this case, there's no real work
to do, so just continue on to the next one.

v2: Update for bblock->prev() API change.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/mesa/drivers/dri/i965/brw_dead_control_flow.cpp

@@ -42,6 +42,10 @@ dead_control_flow_eliminate(backend_shader *s)
 
    foreach_block_safe (block, s->cfg) {
       bblock_t *prev_block = block->prev();
+
+      if (!prev_block)
+         continue;
+
       backend_instruction *const inst = block->start();
       backend_instruction *const prev_inst = prev_block->end();