[ARM] Refactor Thumb2 Mul and Mla instr descs

Recommitting after r274347 was reverted. This patch introduces some
classes to refactor the 3 and 4 register Thumb2 multiplication
instruction descriptions, plus improved tests for some of those
instructions.

Differential Revision: https://reviews.llvm.org/D21929



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@275979 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed