Add a field to the memory access class for a related value.

  The new field in the MemoryAccess allows us to track a value related
  to that access:
    - For real memory accesses the value is the loaded result or the
      stored value.
    - For straigt line scalar accesses it is the access instruction
      itself.
    - For PHI operand accesses it is the operand value.

  We use this value to simplify code which deduced information about the value
  later in the Polly pipeline and was known to be error prone.

Reviewers: grosser, Meinsersbur

Subscribers: #polly

Differential Revision: http://reviews.llvm.org/D12062

llvm-svn: 245213