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.