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

This is the second in the series of patches to enable adding
of machine sched-models for ARM processors easier and compact.
This patch focuses on integer instructions and adds missing
sched definitions.

Reviewers: rovka, rengolin
Differential Revision: https://reviews.llvm.org/D29127

llvm-svn: 293935