[feature/dataflow] Branch exit criteria cleanups

- [x] Make sure dataflow analysis only runs on method bodies that have references to linker-unfriendly methods/fields/or are linker unfriendly themselves.
- [x] Make `StackSlot` a struct, or eliminate `StackSlot` by replacing it with `ValueNode`.
- [ ] Other misc perf measurements and optimizations
- [x] Make sure unsafe reflection pattern reporting can handle multiple reported values (for e.g.`Activator. CreateInstance(cond ? typeof(Foo) : typeof(Bar))`).
- [x] Replace HashHelper with HashCode struct (commit 88b158eb2e468e37c109158256be503e010e1829 in #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
- [x] Make annotation JSON generic - https://github.com/mono/linker/issues/1118
- [ ] Handle assemblies referenced only via reflection - https://github.com/mono/linker/issues/1079 and https://github.com/mono/linker/issues/943
- [ ] Design and implement programmatic way to fully disable method body scanning - https://github.com/mono/linker/issues/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?

Start by choosing one unchecked item and reading the related issues #1079, #943, and #1102; the checklist covers dataflow scanning, reflection, annotations, intrinsics, and allocation costs. Done would require completing one explicitly scoped optimization or design item, but this issue does not define a single acceptance test or a specific code location.

Written by the indexing model from the issue text.