MachineInstr: introduce explicit_operands and implicit_operands ranges

Makes iteration over implicit and explicit machine operands more
explicit (har har). Insipired by code review discussion for r205565.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205680 91177308-0d34-0410-b5e6-96231b3b80d8