[SPARC] Simplify instruction decoder.

After https://reviews.llvm.org/D137653 named sub-operands can be used
in the auto-generated instruction decoders. This allows the
auto-generated decoders to work properly, so all the hand-coded
decoders in the sparc target can be removed.

In some instances, a manually-written decoder had not been implemented
for an instruction, and thus that instruction was not decoded
properly. These have been fixed (and tests added).

Differential Revision: https://reviews.llvm.org/D137727