The parser object can now be created on a worker thread, therefore we shouldn't access
global FLAGs during the constructor. Instead move them to the ParseInfo constructor
and set the parser fields based on these. Also avoid accessing always_opt flags in
bytecode-flags - instead accessing it in ParseInfo and propagating to the bytecode
generator.

Also gets rid of unused kUntrustedCodeMitigations flag in UnoptimizedCompilationInfo

BUG=v8:8582

Change-Id: I6e6fdc8cc7865803cb5f334f652abc0e3e4cb3ce
Reviewed-on: https://chromium-review.googlesource.com/c/1375918Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58316}