- Let MachineInstr ctors add implicit def and use operands. Other operands

  will be inserted before these operands. If the opcode changes (by
  setOpcode), the implicit operands are updated as well.
- Added IsKill, IsDead fields to MachineOperand in preparation for changes
  that move kill / dead info to MachineInstr's.

llvm-svn: 31711