[ScopInfo] Remove dependency of Scop::getLastStmtFor(BB) on getStmtFor(BB). NFC.

We are working towards removing uses of Scop::getStmtFor(BB). In this
patch, we remove dependency of Scop::getLastStmtFor(BB) on
getStmtFor(BB). To do so, we get the list of all statements
corresponding to the BB and then fetch the last one.

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

Differential Revision: https://reviews.llvm.org/D35665

llvm-svn: 308633