[LV] Relax Small Size Reduction Type Requirement

This patch enables small size reductions in which the source types are smaller
than the reduction type (e.g., computing an i16 sum from the values in an i8
array). The previous behavior was to only allow small size reductions if the
source types and reduction type were the same. The change accounts for the fact
that the existing sign- and zero-extend instructions in these cases should
still be included in the cost model.

Differential Revision: http://reviews.llvm.org/D12770

llvm-svn: 247337