sra: Bail out when encountering accesses with negative offsets (PR 96730)

sra: Bail out when encountering accesses with negative offsets (PR 96730)

I must admit I was quite surprised to see that SRA does not disqualify
an aggregate from any transformations when it encounters an offset for
which get_ref_base_and_extent returns a negative offset.  It may not
matter too much because I sure hope such programs always have
undefined behavior (SRA candidates are local variables on stack) but
it is probably better not to perform weird transformations on them as
build ref model with the new build_reconstructed_reference function
currently happily do for negative offsets (they just copy the existing
expression which is then used as the expression of a "propagated"
access) and of course the compiler must not ICE (as it currently does
because the SRA forest verifier does not like the expression).

gcc/ChangeLog:

2020-08-24  Martin Jambor  <mjambor@suse.cz>

	PR tree-optimization/96730
	* tree-sra.c (create_access): Disqualify any aggregate with negative
	offset access.
	(build_ref_for_model): Add assert that offset is non-negative.

gcc/testsuite/ChangeLog:

2020-08-24  Martin Jambor  <mjambor@suse.cz>

	PR tree-optimization/96730
	* gcc.dg/tree-ssa/pr96730.c: New test.