armemu: Fix SADD16

The lo and hi parts of the result were being constructed as a result of hi and lo halfword intermixing from the rm and rn regs. However the lo part of the result should be constructed only from the lo halfwords of rm and rn, and the hi part of the result should only be constructed from the hi halfwords of rm and rn.

See [here](https://github.com/citra-emu/citra/pull/293) for more info (hardware results, etc)