c++, tree: optimize walk_tree_1 and cp_walk_subtrees

c++, tree: optimize walk_tree_1 and cp_walk_subtrees

These functions currently repeatedly dereference tp during the subtree
walks, dereferences which the compiler can't CSE because it can't
guarantee that the subtree walking doesn't modify *tp.

But we already implicitly require that TREE_CODE (*tp) remains the same
throughout the subtree walks, so it doesn't seem to be a huge leap to
strengthen that to requiring *tp remains the same.

So this patch manually CSEs the dereferences of *tp.  This means that a
callback function can no longer replace *tp with another tree (of the
same TREE_CODE) when walking one of its subtrees, but that doesn't sound
like a useful capability anyway.

gcc/cp/ChangeLog:

	* tree.cc (cp_walk_subtrees): Avoid repeatedly dereferencing tp.
	<case DECLTYPE_TYPE>: Use cp_unevaluated and WALK_SUBTREE.
	<case ALIGNOF_EXPR etc>: Likewise.

gcc/ChangeLog:

	* tree.cc (walk_tree_1): Avoid repeatedly dereferencing tp
	and type_p.