Up until now, if one wanted to specify an explicit stack location                                                                                                                                                                                                or register as an operand for an instruction, it had to also be
explicitly associated with a virtual register as a so-called
FixedRegister or FixedStackSlot.

For the implementation of tail calls, the plan is to use the gap
resolver needs to shuffle stack locations from the caller to the
tail-called callee. In order to do this, it must be possible to
explicitly address operand locations on the stack that are not
associated with virtual registers.

This CL introduces ExplictOperands, which can specify a specific
register or stack location that is not associated with virtual
register. This will allow tail calls to specify the target
locations for the necessary stack moves in the gap for the tail
call without the core register allocation having to know about
the target of the stack moves at all.

In the process this CL:
* creates a new Operand kind, ExplicitOperand, with which
  instructions can specify register and stack slots without an
  associated virtual register.
* creates a LocationOperand class from which AllocatedOperand and
  ExplicitOperand are derived and provides a common interface to
  get Register, DoubleRegister and spill slot information.
* removes RegisterOperand, DoubleRegisterOperand,
  StackSlotOperand and DoubleStackSlotOperand, they are subsumed
  by LocationOperand.
* addresses a cleanup TODO in AllocatedOperand to reduce the
  redundancy of AllocatedOperand::Kind by using machine_type() to
  determine if an operand corresponds to a general purpose or
  double register.

BUG=v8:4076
LOG=n

Review URL: https://codereview.chromium.org/1389373002

Cr-Commit-Position: refs/heads/master@{#31603}