[turbofan] cleanup InstructionOperand a little

[turbofan] cleanup InstructionOperand a little

- ConstantOperand was using a too-small field too store its virtual register
- drop ConvertTo, replace it with simple copy
- split AllocatedOperand off from Immediate and Constant to make assignment clearer, also paving the way for small Immediates
- put zone first in *Operand::New
- driveby: drop delayed ssa deconstruction experiment

R=titzer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1050803002

Cr-Commit-Position: refs/heads/master@{#27692}