pco: Fix encoding of branch to an empty block

When calculating the relative offset for a branch the pco_first_igrp
function is used to find the first instruction of a block.

However if the block is empty the function does not return NULL as it
description implies but returns a pointer to the list head which is not a
valid node. Using this leads to a garbage relative offset been calculated
which leads to unexpected behaviour.

Fix is to add a check for the list been empty and return NULL (the same
issue also exists in pco_last_igrp). This leads to the calling function,
pco_cf_node_offset, searching for the next none empty block which is the
expected behaviour.

Fix deqp:
dEQP-VK.graphicsfuzz.cov-two-nested-loops-switch-case-matrix-array-increment
dEQP-VK.graphicsfuzz.stable-binarysearch-tree-false-if-discard-loop

Signed-off-by: Nick Hamilton <nick.hamilton@imgtec.com>
Reviewed-by: Simon Perretta <simon.perretta@imgtec.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/39287>

src/imagination/pco/pco_internal.h

@@ -1534,6 +1534,9 @@ static inline pco_instr *pco_prev_instr(pco_instr *instr)
  */
 static inline pco_igrp *pco_first_igrp(pco_block *block)
 {
+   if (list_is_empty(&block->instrs))
+      return NULL;
+
    return list_first_entry(&block->instrs, pco_igrp, link);
 }
 
@@ -1545,6 +1548,9 @@ static inline pco_igrp *pco_first_igrp(pco_block *block)
  */
 static inline pco_igrp *pco_last_igrp(pco_block *block)
 {
+   if (list_is_empty(&block->instrs))
+      return NULL;
+
    return list_first_entry(&block->instrs, pco_igrp, link);
 }