The flag must be used only by CodeStubAssemblerGraphsCorrectness cctest for now
and once all the verification issues are fixed the flag will be enabled in debug
mode by default.

This CL also relaxes some checks for code stub graphs and fixes some issues in the stubs.

BUG=

Review-Url: https://codereview.chromium.org/2558653002
Cr-Commit-Position: refs/heads/master@{#41547}