This removes a restriction from full-codegen that limited the usability
of the --debug-code flag to only no-snap configurations. The reasoning
for the restriction would still hold, if we ever put full-codegen code
into the snapshot, which we don't. Also there already are several places
in full-codegen that queried the FLAG_debug_code directly, a more
reliable mechanism will be needed if we snapshot full code.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/1722593002

Cr-Commit-Position: refs/heads/master@{#34189}