x86: process instruction operands for .insn

x86: process instruction operands for .insn

Deal with register and memory operands; immediate operands will follow
later, as will the handling of EVEX embedded broadcast and EVEX Disp8
scaling.

Note that because we can't really know how to encode their use, %cr8 and
up cannot be used with .insn outside of 64-bit mode. Users would need to
specify an explicit LOCK prefix in combination with %cr0 etc.