c++: Optimize away NULLPTR_TYPE comparisons [PR101443]

c++: Optimize away NULLPTR_TYPE comparisons [PR101443]

Comparisons of NULLPTR_TYPE operands cause all kinds of problems in the
middle-end and in fold-const.c, various optimizations assume that if they
see e.g. a non-equality comparison with one of the operands being
INTEGER_CST and it is not INTEGRAL_TYPE_P (which has TYPE_{MIN,MAX}_VALUE),
they can build_int_cst (type, 1) to find a successor.

The following patch fixes it by making sure they don't appear in the IL,
optimize them away at cp_fold time as all can be folded.

Though, I've just noticed that clang++ rejects the non-equality comparisons
instead, foo () > 0 with
invalid operands to binary expression ('decltype(nullptr)' (aka 'nullptr_t') and 'int')
and foo () > nullptr with
invalid operands to binary expression ('decltype(nullptr)' (aka 'nullptr_t') and 'nullptr_t')

Shall we reject those too, in addition or instead of parts of this patch?
If so, wouldn't this patch be still useful for backports, I bet we don't
want to start reject it on the release branches when we used to accept it.

2021-07-15  Jakub Jelinek  <jakub@redhat.com>

	PR c++/101443
	* cp-gimplify.c (cp_fold): For comparisons with NULLPTR_TYPE
	operands, fold them right away to true or false.

	* g++.dg/cpp0x/nullptr46.C: New test.

(cherry picked from commit 7094a69bd62a14dfa311eaa2fea468f221c7c9f3)