[ARM] Add 2-operand assembly aliases for Thumb1 ADD/SUB

This adds 2-operand assembly aliases for these instructions:
  add r0, r1    =>   add r0, r0, r1
  sub r0, r1    =>   sub r0, r0, r1

Previously this syntax was only accepted for Thumb2 targets, where the
wide versions of the instructions were used.

This patch allows the 2-operand syntax to be used for Thumb1 targets,
and selects the narrow encoding when it is used for Thumb2 targets.

Differential revision: https://reviews.llvm.org/D37377

llvm-svn: 312321