[polly] [ScopInfo] Don't pre-compute the name of the Scop's region.

This gets very expensive for basic blocks which don't have a name: it
calls printAsOperand, which numbers the entire module. We don't
normally need the name anyway, though; it's only used for debug dumps,
so don't compute it by default.

Differential Revision: https://reviews.llvm.org/D44946

llvm-svn: 328666