This is in preparation for introducing more specialized
CodeStubAssembler subclasses. The state object can be handed
around, while the Assembler instances are temporary-scoped.

BUG=v8:5628

Original review: https://codereview.chromium.org/2498073002/

Review-Url: https://codereview.chromium.org/2502293002
Cr-Commit-Position: refs/heads/master@{#41028}