tree-optimization/108522 Use component_ref_field_offset

tree-optimization/108522 Use component_ref_field_offset

Instead of using TREE_OPERAND (expr, 2) directly, use
component_ref_field_offset instead, which does scaling for us.  The
function also substitutes PLACEHOLDER_EXPRs but it is not relevant for
tree-object-size.

gcc/ChangeLog:

	PR tree-optimization/108522
	* tree-object-size.cc (compute_object_offset): Make EXPR
	argument non-const.  Call component_ref_field_offset.

gcc/testsuite/ChangeLog:

	PR tree-optimization/108522
	* gcc.dg/builtin-dynamic-object-size-0.c (DEFSTRUCT): New
	macro.
	(test_dynarray_struct_member_b, test_dynarray_struct_member_c,
	test_dynarray_struct_member_d,
	test_dynarray_struct_member_subobj_b,
	test_dynarray_struct_member_subobj_c,
	test_dynarray_struct_member_subobj_d): New tests.
	(main): Call them.

Signed-off-by: Siddhesh Poyarekar <siddhesh@gotplt.org>