[ARM] Small refactor of Thumb2 SMLA insts
[ARM] Small refactor of Thumb2 SMLA insts

Follow up to r276624. Changes bits 22-20 to be parameters to
instruction class.

Differential Revision: https://reviews.llvm.org/D22562



git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@276626 91177308-0d34-0410-b5e6-96231b3b80d8

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