calls: Introduce cxx17_empty_base_field_p [PR94383]

calls: Introduce cxx17_empty_base_field_p [PR94383]

As multiple targets are affected apparently, I believe at least
aarch64, arm, powerpc64le, s390{,x} and ia64,
I think we should have a middle-end predicate for this, so that if we need
to tweak it, we can do it in one spot.

2020-04-22  Jakub Jelinek  <jakub@redhat.com>

	PR target/94383
	* calls.h (cxx17_empty_base_field_p): Declare.
	* calls.c (cxx17_empty_base_field_p): Define.