[ScopInfo] Get a list of statements for a region node. NFC.

When constructing a schedule true and there are multiple statements for
a basic block, create a sequence node for these statements.

Contributed-by: Nandini Singhal <cs15mtech01004@iith.ac.in>

Differential Revision: https://reviews.llvm.org/D35679

llvm-svn: 308635