[ScopInfo] Remove usage of isl_set_n_basic_set()

Summary: This patch aims to remove the usage of old C-styled isl functions (in this case `isl_set_n_basic_set()`) in favor of new C++ isl interface based methods in `ScopInfo.cpp`.

Patch by Sahil Yerawar

Differential Revision: https://reviews.llvm.org/D46935

llvm-svn: 332471