fold-const: Avoid (cast) ((cast2) x p+ y) folding for -fsanitize=alignment [PR98206]

fold-const: Avoid (cast) ((cast2) x p+ y) folding for -fsanitize=alignment [PR98206]

The following testcase is diagnosed by UBSan as invalid, even when it is
valid.
We have a derived type Base2 at offset 1 with alignment 1 and do:
(const Derived &) ((const Base2 *) this + -1)
but the folder before ubsan in the FE gets a chance to instrument it
optimizes that into:
(const Derived &) this + -1
and so we require that this has 8-byte alignment which Derived class needs.

Fixed by avoiding such an optimization when -fsanitize=alignment is in
effect if it would affect the alignments (and guarded with !in_gimple_form
because we don't really care during GIMPLE, though pointer conversions are
useless then and so such folding isn't needed very much during GIMPLE).

2020-12-31  Jakub Jelinek  <jakub@redhat.com>

	PR c++/98206
	* fold-const.c: Include asan.h.
	(fold_unary_loc): Don't optimize (ptr_type) (((ptr_type2) x) p+ y)
	into ((ptr_type) x) p+ y if sanitizing alignment in GENERIC and
	ptr_type points to type with higher alignment than ptr_type2.

	* g++.dg/ubsan/align-4.C: New test.