[ARM][ParallelDSP] Change the search for smlads

    
Two functional changes have been made here:
- Now search up from any add instruction to find the chains of
  operations that we may turn into a smlad. This allows the
  generation of a smlad which doesn't accumulate into a phi.
- The search function has been corrected to stop it falsely searching
  up through an invalid path.
    
The bulk of the changes have been making the Reduction struct a class
and making it more C++y with getters and setters.

Differential Revision: https://reviews.llvm.org/D61780

llvm-svn: 365740