ARM assembly parsing and encoding for RFE instruction.

Fill in the missing fixed bits and the register operand bits of the instruction
encoding. Refactor the definition to make the mode explicit, which is
consistent with how loads and stores are normally represented and makes
parsing much easier. Add parsing aliases for pseudo-instruction variants.
Update the disassembler for the new representations. Add tests for parsing and
encoding.

llvm-svn: 136479