[turbofan] cleanup ParallelMove

- make ParallelMove into a ZoneVector, removing an annoying level of indirection
- make MoveOperands hold InstructionOperands instead of pointers, so there's no more operand aliasing for moves
- opens up possibility of storing MachineType in allocated operands

R=bmeurer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1081373002

Cr-Commit-Position: refs/heads/master@{#27842}