[ARM][LowOverheadLoops] Liveouts and reductions

Remove the code that tried to look for reduction patterns, since the
vectorizer and isel can now produce predicated arithmetic instructios
within the loop body. This has required some reorganisation and fixes
around live-out and predication checks, as well as looking for cases
where an input/output is initialised to zero.

Differential Revision: https://reviews.llvm.org/D86613