[NFC][ARM[ParallelDSP] Cleanup of BinOpChain

- Remove some unused typedefs.
- Rename BinOpChain struct to MulCandidate.
- Remove the size method of MulCandidate.
- Store only the first input of the ValueList provided to
  MulCandidate, as it's the only value we care about. This means we
  don't have to perform any ugly (and unnecessary) iterations of the 
  list later on.

llvm-svn: 367208