This patch implements the original suggestion for fixing PR 81635:
use range info in split_constant_offset to see whether a conversion
of a wrapping type can be split.  The range info problem described in:

    https://gcc.gnu.org/ml/gcc-patches/2017-08/msg01002.html

seems to have been fixed.

The patch is part 1.  There needs to be a follow-on patch to handle:

  for (unsigned int i = 0; i < n; i += 4)
    {
      ...[i + 2]...
      ...[i + 3]...

which the old SCEV test handles, but which the range check doesn't.
At the moment we record that the low two bits of "i" are clear,
but we still end up with a maximum range of 0xffffffff rather than
0xfffffffc.

2018-01-31  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	PR tree-optimization/81635
	* tree-data-ref.c (split_constant_offset_1): For types that
	wrap on overflow, try to use range info to prove that wrapping
	cannot occur.

gcc/testsuite/
	PR tree-optimization/81635
	* gcc.dg/vect/bb-slp-pr81635-1.c: New test.
	* gcc.dg/vect/bb-slp-pr81635-2.c: Likewise.

From-SVN: r257296