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It turned out to be a trivial change to support profiled castclass as an addition to profiled isinst.

To move non-profiled casts I will have to add a few additional things:

1. In some cases we don't need the fallback call at all, e.g. obj is string because string is sealed
2. In some cases the fallback call should be marked as no-return (and its block converted to BBJ_THROW)
3. getExactClasses for NativeAOT
   ...

PTAL @jakobbotsch @dotnet/jit-contrib Build failures are known. Diffs