CodeAssemblerLabel's destructor was not calling RawMachineLabel's destructor, because label_ is zone-allocated. RawMachineLabel's destructor contains a DCHECK that would make debugging easier. If the DCHECK is not triggered, things will go awry in the register allocation phase, making it harder to debug the issue.

BUG=

Review-Url: https://codereview.chromium.org/2641863002
Cr-Commit-Position: refs/heads/master@{#42461}