Don't use emit_parts_list for regular operations.

Using emit_parts_list does not make sense for any
operation except Ialloc, where we can delay the
computation of simple expression until the allocation
is done. So we try to avoid using emit_parts_list for
regular operations, where the special treatment of
non simple expression conflicts with our special
instruction selection on arm (and arm64). For example
we cannot merge the add and shift operation required
for tagging as soon as there is at least one non
simple expression involved, even though both Cadd
and Clsl do not care at all whether its arguments are
simple or not.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14292 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02