This changes the compilation pipeline so that mixed stacks are allowed
when bytecode is preserved. This means there can be activations of both,
"baseline" as well as "unoptimized" code active on the stack at the same
time for any single given function.

R=rmcilroy@chromium.org
BUG=v8:4280

Review-Url: https://codereview.chromium.org/2267693002
Cr-Commit-Position: refs/heads/master@{#38809}