This is to fix a bug in the bytecode graph builder. This cl adds a new merge
node before we copy the environment on conditional/unconditional jumps. Since
these environments could be merged later, we add a place holder merge so that
the control dependencies are correctly merged. If we do not have a merge node
we may incorrectly merge the dependencies into the previous block.
For ex: test-run-variables/ContextStoreVariables in cctests.

BUG=v8:4280
LOG=N

Review URL: https://codereview.chromium.org/1641143002

Cr-Commit-Position: refs/heads/master@{#33591}