- InstructionAttr fields interleave bools and ints, which mean the size of the class is larger than it needs to be. Rearranging so that bool and ints are together gets sizeof(InstructionAttr) down from 48 to 40, with slightly better code gen.

- _is_managed only used on 32-bit

- code gen for *_needs_rex improve if replacing reg != (x)noreg
  with reg->is_valid(), since the expression compiles down into a
  range checks (fewer branches)