[ARM] bottom-top mul support in ARMParallelDSP

Previously reverted in rL343082.

Original commit message:

On failing to find sequences that can be converted into dual macs,
try to find sequential 16-bit loads that are used by muls which we
can then use smultb, smulbt, smultt with a wide load.

Differential Revision: https://reviews.llvm.org/D51983

llvm-svn: 344693