We use Schedule::EnsureDeferredCodeSingleEntryPoint as a helper for 
hand-crafted builtin code, to ensure deferred code isn't entered from a
mix of deferred and non-deferred code (invariant required for hot/cold
allocation, or "splintering").

When we create a "merger" block, it may be the case that the original 
block had a few phi operands. Those need to be moved as well.

This bug was uncovered by both v8:6390, and, earlier, by v8:5998. We 
fixed the earlier one by authoring a the builtin to avoid the need for
EnsureDeferredCodeSingleEntryPoint. I proposed earlier an alternative 
where we'd replace the Ensure... method with a Verify, and throw early
when the builtin is assembled, however, we may want to maintain the 
slightly higher level DSL for authoring builtins, and perform such 
graph adjustments for the lower level constraints afterwards, hence 
this current CL.

Bug: v8:5998 v8:6390
Change-Id: Ia3143f7a66904fe480d8edb5b52bf915b8d185dc
Reviewed-on: https://chromium-review.googlesource.com/505264
Commit-Queue: Mircea Trofin <mtrofin@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45387}