Update ISL to isl-0.15-3-g532568a

This version adds small integer optimization, but is not active by
default. It will be enabled in a later commit.
    
The schedule-fuse=min/max option has been replaced by the
serialize-sccs option. Adapting Polly was necessary, but retaining the
name polly-opt-fusion=min/max.

Differential Revision: http://reviews.llvm.org/D10505

Reviewers: grosser
llvm-svn: 240027