We recently fixed a bug in "sparsifying" such reductions, since
it incorrectly changed this into reductions over stored elements
only , which only works for add/sub/or/xor. However, we still want
to be able to "sparsify" the reductions even in the general case,
and this is a first step by rewriting them into a custom reduction
that feeds in the implicit zeros. NOTE HOWEVER, that in the long run
we want to do this better and feed in any implicit zero only ONCE
for efficiency.
Reviewed By: Peiming
Differential Revision: https://reviews.llvm.org/D152580
80fe3168b5