- Removes set_virtual_register method. InstructionOperands are immutable.
- Adds a new ctor to copy an UnallocatedOperand with a new vreg.
- Removes some DCHECKs in UnallocatedOperand that are always true. To
  make sure, make UnallocatedOperand final.
- Cleans up some comments on UnallocatedOperand Lifetime enum.

BUG=v8:6325

Review-Url: https://codereview.chromium.org/2897203002
Cr-Commit-Position: refs/heads/master@{#45533}