Phi creation in the code assembler was dependent on the addresses
of the CodeAssemblerVariable::Impl object. This caused non-determinism
in mksnapshot which sometimes occurred on Windows.

This CL adds IDs to CodeAssemblerVariable::Impl objects and ensures
all iterations are done in ID order instead of object address order.


Change-Id: I2b370dc5153202be864a5c13289e70f5ebd59e2e
Bug: v8:8391
Reviewed-on: https://chromium-review.googlesource.com/c/1319749
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57276}