[feature/dataflow] Branch exit criteria cleanups

[MichalStrehovsky]

• Make sure dataflow analysis only runs on method bodies that have references to linker-unfriendly methods/fields/or are linker unfriendly themselves.
• Make StackSlot a struct, or eliminate StackSlot by replacing it with ValueNode.
• Other misc perf measurements and optimizations
• Make sure unsafe reflection pattern reporting can handle multiple reported values (for e.g.Activator. CreateInstance(cond ? typeof(Foo) : typeof(Bar))).
• Replace HashHelper with HashCode struct (commit 88b158e in [feature/dataflow] Add dataflow analysis prototype #994 tried to do it but hit trouble in tests)
• Remove usage of lambdas which require closure - requires allocation on every call
• Measure and optimize the number of methods full method body scanning runs on
• Make annotation JSON generic - Make the supplementary custom attribute annotations general purpose #1118
• Handle assemblies referenced only via reflection - [feature/dataflow] Add a pre-pass to scan type name strings in the app #1079 and Reflection patterns can not resolve assemblies outside of the reference closure #943
• Design and implement programmatic way to fully disable method body scanning - [feature/dataflow] Design and implement programmatic way to disable method body scanning #1102
• Optimize the way we detect intrinsic - instead of switch on every referenced method, look these up upfront and build annotation cache
• Investigate a better way to compute annotations - currently done per-type (so referencing one thing on a type forces full annotation cache to be build for the entire type). Can we do it more incrementally?

[sbomer]

We have some testing of multiple reported values in GetTypeDataFlow, and I'm adding a few more in #1090.

[vitek-karas]

#1099 implements only running analysis on interesting methods.