X86: When decoding a memory only inst, fault on reg encodings, don't assert.

X86: When decoding a memory only inst, fault on reg encodings, don't assert.

This change makes the decoder figure out if an instruction that only supports
memory is using a register encoding and decodes directly to "Unknown" which will
behave appropriately. This prevents other parts of the instruction creation
process from seeing the mismatch and asserting.