[ScopBuilder] Pass ScopStmts around instead of BasicBlocks. NFC.

During the construction of MemoryAccesses in ScopBuilder, BasicBlocks
were used in function parameters, assuming that the ScopStmt an be
directly derived from it. This won't be true anymore once we split
BasicBlocks into multiple ScopStmt. As a preparation for such a change
in the future, we instead pass the ScopStmt and avoid the use of
getStmtFor().

There are two occasions where a kind of mapping from BasicBlock to
ScopStmt is still required.

1. Get the statement representing the incoming block of a `PHINode`
   using `getLastStmtOf`.

2. One statement is required to write a scalar to be readable by those
   which need it. This is most often the statement which contains its
   definition, which we get using `getStmtFor(Instruction*)`.

Differential Revision: https://reviews.llvm.org/D34369

llvm-svn: 306132