Fix GC_is_valid_displacement and GC_is_visible for non-small objects

Fix GC_is_valid_displacement and GC_is_visible for non-small objects
(a cherry-pick of commit 77f4ff04 from 'master')

* ptr_chck.c (GC_is_valid_displacement): Remove redundant
IS_FORWARDING_ADDR_OR_NIL(hhdr) call if GC_all_interior_pointers.
* ptr_chck.c (GC_is_valid_displacement): Do not goto fail if
p+sz-offset > h+1 but IS_FORWARDING_ADDR_OR_NIL(HDR(h+1)).
* ptr_chck.c (GC_is_visible): Transform comment about GC_base to
a TODO item; set hhdr to HDR(base) instead of HDR(p)
if HBLKPTR(base)!=HBLKPTR(p).