[ARM] Classification Improvements to ARM Sched-Models. NFCI.

This patch adds missing sched classes for Thumb2 instructions.
This has been missing so far, and as a consequence, machine
scheduler models for individual sub-targets have tended to
be larger than they needed to be. These patches should help
write schedulers better and faster in the future
for ARM sub-targets.

Reviewer: Diana Picus
Differential Revision: https://reviews.llvm.org/D29953

llvm-svn: 295811