Update isl-cpp bindings

We upstreamed the export of isl_val_2exp, to the official cpp bindings.
In this process, we concluded that pow2 is a better and more widely used
name for this functionality. Hence, both the official isl-cpp bindings
and our derived variant use now the term pow2.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@339312 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed