i965: Allow exec_list sentinels as arguments to insert functions.

i965: Allow exec_list sentinels as arguments to insert functions.

To insert an instruction at the end of a basic block, we typically do
something like

   inst = block->last_non_control_flow_inst();
   inst->insert_after(block, new_inst);

But blocks can consist of a single control flow instruction, so inst
will actually be the exec_list's head sentinel. We shouldn't use it as
if it were a regular instruction, but it is safe to insert something after
it.

This patch avoids assert-failing because an exec_list sentinel wasn't in
the basic block's instruction list.

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