core: Synchronize tree-cst & wide-int caching expectations

core: Synchronize tree-cst & wide-int caching expectations

I fell over an ICE where wide_int_to_type_1's expectations of pointer
value caching didn't match that of cache_integer_cst's behaviour.  I
don't know why it only exhibited on the modules branch, but it seems
pretty wrong.  This patch matches up the behaviours and adds a comment
about that.

	gcc/
	* tree.c (cache_integer_cst): Fixup pointer caching to match
	wide_int_to_type_1's expectations.  Add comment.