[ScopBuilder] Avoid use of getStmtFor(BB). NFC.

Since there will be no more a 1:1 correspondence between statements
and basic blocks, we would like to get rid of the method getStmtFor(BB)
and its uses. Here we remove one of its uses in ScopBuilder by fetching
the statement in which the instruction lies.

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

Differential Revision: https://reviews.llvm.org/D35610

llvm-svn: 308610