[SI] Create Scop Name lazily

Summary: Creating the Scop name is expensive, because creating the
Region name it's derived from is expensive. So create the name lazily,
because getName() is actually called rarely.

This is a reiteration of r328666, which introduced a use-after-free and
got reverted in r331363.

Differential Revision: https://reviews.llvm.org/D46868

llvm-svn: 332359