member descriptor ics

trying to fill in rewriting holes... for this one I had to some support for doubles and some `mov` variants in the assembler and rewriter. Kind of hacky/lazy...
Hmm I'm not sure this is right -- I think an 0x40 REX prefix still can have meaning.  For example I think you need to specify it to use certain 1-byte register parts: http://wiki.osdev.org/X86-64_Instruction_Encoding#REX_prefix

ie I think this handling (don't emit rex if unnecessary) has to be handled by the instruction-specific function.