[ARM][THUMB2] Allow emitting T3 types of add and sub

Summary:
This patch allows to emit thumb2 add and sub
instructions with 12 bit immediates in the
emitT2RegPlusImmediate function.
- Splitting parts of the D70680

Reviewers: eli.friedman, olista01, efriedma

Reviewed By: efriedma

Subscribers: efriedma, kristof.beyls, hiraditya, dmgreen, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71361