data-ref: Tighten index-based alias checks [PR99726]

data-ref: Tighten index-based alias checks [PR99726]

create_intersect_range_checks_index tries to create a runtime
alias check based on index comparisons.  It looks through the
access functions for the two DRs to find a SCEV for the loop
that is being versioned and converts a DR_STEP-based check
into an index-based check.

However, there isn't any reliable sign information in the types,
so the code expects the value of the IV step (when interpreted as
signed) to be negative iff the DR_STEP (when interpreted as signed)
is negative.

r10-4762 added another assert related to this assumption and the
assert fired for the testcase in the PR.  The sign of the IV step
didn't match the sign of the DR_STEP.

I think this is actually showing what was previously a wrong-code bug.
The signs didn't match because the DRs contained *two* access function
SCEVs for the loop being versioned.  It doesn't look like the code
is set up to deal with this, since it checks each access function
independently and treats it as the sole source of DR_STEP.

The patch therefore moves the main condition out of the loop.
This also has the advantage of not building a tree for one access
function only to throw it away if we find an inner function that
makes the comparison invalid.

gcc/
	PR tree-optimization/99726
	* tree-data-ref.c (create_intersect_range_checks_index): Bail
	out if there is more than one access function SCEV for the loop
	being versioned.

gcc/testsuite/
	PR tree-optimization/99726
	* gcc.target/i386/pr99726.c: New test.