Update to isl-0.16.1-20-gee54b48

This includes some (optional) improvements to the isl scheduler, which we do not
use yet, as well as a fix for a bug previously also affecting Polly:

commit 662ee9b7d45ebeb7629b239d3ed43442e25bf87c
Author: Sven Verdoolaege <skimo@kotnet.org>
Date:   Mon Jan 25 16:59:32 2016 +0100

    isl_basic_map_realign: perform Gaussian elimination on result

    Many parts of isl assume that Gaussian elimination has been
    applied to the equality constraints.  In particular singleton_extract_point
    makes this assumption.  The input to singleton_extract_point
    may have undergone parameter alignment.  This parameter alignment
    (ultimately performed by isl_basic_map_realign) therefore
    needs to make sure the result preserves this property

llvm-svn: 259757