vect: Fix access size alignment assumption [PR115192]

vect: Fix access size alignment assumption [PR115192]

create_intersect_range_checks checks whether two access ranges
a and b are alias-free using something equivalent to:

  end_a <= start_b || end_b <= start_a

It has two ways of doing this: a "vanilla" way that calculates
the exact exclusive end pointers, and another way that uses the
last inclusive aligned pointers (and changes the comparisons
accordingly).  The comment for the latter is:

      /* Calculate the minimum alignment shared by all four pointers,
	 then arrange for this alignment to be subtracted from the
	 exclusive maximum values to get inclusive maximum values.
	 This "- min_align" is cumulative with a "+ access_size"
	 in the calculation of the maximum values.  In the best
	 (and common) case, the two cancel each other out, leaving
	 us with an inclusive bound based only on seg_len.  In the
	 worst case we're simply adding a smaller number than before.

The problem is that the associated code implicitly assumed that the
access size was a multiple of the pointer alignment, and so the
alignment could be carried over to the exclusive end pointer.

The testcase started failing after g:9fa5b473b5b8e289b6542
because that commit improved the alignment information for
the accesses.

gcc/
	PR tree-optimization/115192
	* tree-data-ref.cc (create_intersect_range_checks): Take the
	alignment of the access sizes into account.

gcc/testsuite/
	PR tree-optimization/115192
	* gcc.dg/vect/pr115192.c: New test.