[compiler] Distingush optional/extraneous deps

In ValidateExhaustiveDependencies, I previously changed to allow extraneous dependencies as long as they were non-reactive. Here we make that more precise, and distinguish between values that are definitely referenced in the memo function but optional as dependencies vs values that are not even referenced in the memo function. The latter now error as extraneous even if they're non-reactive. This also turned up a case where constant-folded primitives could show up as false positives of the latter category, so now we track manual deps which quality for constant folding and don't error on them.