[X86] Teach the disassembler to use %eiz/%riz instead of NoRegister when the...

[X86] Teach the disassembler to use %eiz/%riz instead of NoRegister when the SIB byte is present, but doesn't encode an index register and there was another shorter encoding that would achieve the same result.

The %eiz/%riz are dummy registers that force the encoder to emit a SIB byte when it normally wouldn't. By emitting them in the disassembly output we ensure that assembling the disassembler output would also produce a SIB byte.

This should match the behavior of objdump from binutils.

llvm-svn: 335768