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>

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

@@ -993,7 +993,8 @@ adjust_later_block_ips(bblock_t *start_block, int ip_adjustment)
 void
 backend_instruction::insert_after(bblock_t *block, backend_instruction *inst)
 {
-   assert(inst_is_in_block(block, this) || !"Instruction not in block");
+   if (!this->is_head_sentinel())
+      assert(inst_is_in_block(block, this) || !"Instruction not in block");
 
    block->end_ip++;
 
@@ -1005,7 +1006,8 @@ backend_instruction::insert_after(bblock_t *block, backend_instruction *inst)
 void
 backend_instruction::insert_before(bblock_t *block, backend_instruction *inst)
 {
-   assert(inst_is_in_block(block, this) || !"Instruction not in block");
+   if (!this->is_tail_sentinel())
+      assert(inst_is_in_block(block, this) || !"Instruction not in block");
 
    block->end_ip++;