[ScopInfo] Introduce list of statements in Scop::StmtMap. NFC.

Once statements are split, a BasicBlock will comprise of multiple
statements. To prepare for this change in future, we introduce a list
of statements in the statement map.

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

Differential Revision: https://reviews.llvm.org/D35301

llvm-svn: 308318