[ScopInfo] Do not use ScopStmt in Domain derivation of ScopInfo. NFC

ScopStmts were being used in the computation of the Domain of the SCoPs
in ScopInfo. Once statements are split, there will not be a 1-to-1
correspondence between Stmts and Basic blocks. Thus this patch avoids
the use of getStmtFor() by creating a map of BB to InvalidDomain and
using it to compute the domain of the statements.

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

Differential Revision: https://reviews.llvm.org/D33942

llvm-svn: 306667