[regexp] Move helper functions to custom assembler

This moves all helper functions to the RegExpBuiltinsAssembler together
with some follow-up cleanups for:

* consistent function names,
* consistent parameter names and order,
* removal of global CVariable and CLabel typedefs.

BUG=v8:5339

Review-Url: https://codereview.chromium.org/2554453003
Cr-Commit-Position: refs/heads/master@{#41493}