[PATCH GCC][3/3]Refine CFG and bound information for split loops

[Bin Cheng]

Hi,
This is a rework of patch at 
https://gcc.gnu.org/ml/gcc-patches/2017-06/msg01037.html.
The new patch doesn't try to handle all cases, instead, it only handles obvious 
cases.
It also tries to add tests illustrating different cases handled.
Bootstrap and test for patch set on x86_64 and AArch64.  Comments?

Thanks,
bin
2017-10-16  Bin Cheng  <bin.ch...@arm.com>

        * tree-ssa-loop-split.c (compute_new_first_bound): New parameter.
        Compute and return bound information for the second split loop.
        (adjust_loop_split): New function.
        (split_loop): Update use and call above function.

gcc/testsuite/ChangeLog
2017-10-16  Bin Cheng  <bin.ch...@arm.com>

        * gcc.dg/loop-split-1.c: New test.
        * gcc.dg/loop-split-2.c: New test.
        * gcc.dg/loop-split-3.c: New test.