[ARM][NFC] ParallelDSP reorganisation

In preparing to allow ARMParallelDSP pass to parallelise more than
smlads, I've restructed some elements:

- The ParallelMAC struct has been renamed to BinOpChain.
- The BinOpChain struct holds two value lists: LHS and RHS, as well
  as inheriting from the OpChain base class.
- The OpChain struct holds all the values of the represented chain
  and has had the memory locations functionality inserted into it.
- ParallelMACList becomes OpChainList and it now holds pointers
  instead of objects.

Differential Revision: https://reviews.llvm.org/D49020

llvm-svn: 337701