A CompilationInfo constructed from just an Isolate* and a Zone* is in
weird an inconsistent state (calling e.g. flags() on it will crash),
so we need to avoid them. This CL removes almost all of them, the
remaining 2 call sites in (for testing only) will be handled in a
separate CL. Things which have been changed:

  * Linkage is basically a decorator for CallDescriptor now.

  * ChangeLowering doesn't need Linkage at all.

  * JSGenericLowering doesn't need a full CompilationInfo*, just a
    single flag.

  * JSContextSpecializer doesn't need the full CompilationInfo, just a
    Context.

  * Removed unused CompilationInfo from SimplifiedLoweringTester.

This nicely decouples things already a bit more, but there's still
work to do...

Review URL: https://codereview.chromium.org/899803003

Cr-Commit-Position: refs/heads/master@{#26580}