[ARM] Small reorganisation in ARMParallelDSP

A few code movement things:

- AreSymmetrical is now a method of BinOpChain.
- Created a lambda in CreateParallelMACPairs to reduce loop nesting.
- A Reduction object now gets pasted in a couple of places instead,
  including CreateParallelMACPairs so it doesn't need to return a
  value.
I've also added RecordSequentialLoads, which is run before the
transformation begins, and caches the interesting loads. This can then
be queried later instead of cross checking many load values.

Differential Revision: https://reviews.llvm.org/D54254

llvm-svn: 346479