c++: Expr pack expansion equality [pr94454]

We were not comparing expression pack expansions correctly. We could
consider distinct expansions equal and creating two, apparently equal,
specializations that would sometimes collide.  cp_tree_operand_length
says a pack has 1 operand (for mangling), whereas it actually has 3,
but only two of which are significant for equality.  We must special
case that in cp_tree_equal.  That new code matches the hasher and the
type_pack_expansion case in structural_comp_types.

	* tree.c (cp_tree_equal): [TEMPLATE_ID_EXPR, default] Refactor.
	[EXPR_PACK_EXPANSION]: Add.