Fix 'OMP_CLAUSE_TILE' operands handling in 'gcc/tree.c:walk_tree_1'

Fix 'OMP_CLAUSE_TILE' operands handling in 'gcc/tree.c:walk_tree_1'

In r245300 (commit 02889d23ee3b02854dff203dd87b9a25e30b61b4)
"OpenACC tile clause support" that one had changed to three operands,
similar to 'OMP_CLAUSE_COLLAPSE'.

There is no (existing) test case where this seems to matter (likewise
for 'OMP_CLAUSE_COLLAPSE'), but it's good to be consistent.

	gcc/
	* tree.c (walk_tree_1) <OMP_CLAUSE_TILE>: Handle three operands.

(cherry picked from commit 92dc5d844a2088db79bc4521be3ecb4e2f284444)