Instead of allocating and embedding certain heap numbers into the code
during code assembly, emit dummies but record the allocation requests.
Later then, in Assembler::GetCode, allocate the heap numbers and patch
the code by replacing the dummies with the actual objects. The
RelocInfos for the embedded objects are already recorded correctly when
emitting the dummies.

R=jarin@chromium.org
BUG=v8:6048

Review-Url: https://codereview.chromium.org/2900683002
Cr-Commit-Position: refs/heads/master@{#45635}