Do not add accesses for intra-ScopStmt scalar def-use chains

When pulling a llvm::Value to be written as a PHI write, the former
code did only check whether it is within the same basic block, but it
could also be the same non-affine subregion. In that case some 
unecessary pair of MemoryAccesses would have been created.

Two unit test were explicitely checking for the unecessary writes,
including the comments that the writes are unecessary. 

llvm-svn: 250411