Removes the need for a CanonicalHandleScope for parsing and renumbering
phases when using Ignition. Since AST strings are canonicalized by the
AST value factory, we only need to make sure we use the same canonical
handles for any other handles we add to the bytecode generator.

This avoids a regression when enabling Ignition for all Turbofan code, and
improves CodeLoad on for Ignition by about 5%.

BUG=v8:4280

Review-Url: https://codereview.chromium.org/2448323004
Cr-Commit-Position: refs/heads/master@{#40595}