Minor improvements for AddHashElement

[onyxmaster]

Hello!

There are small performance improvements worth about 13% speedup for .NET 8+9 on the AddElementHash timing in order of effect:

• elide extra bounds check for lookupDense using ref var
• tune branching using explicit maximum comparison and mutation checks, especially for repeated additions
• remove the need for devirtualization by replacing private field type of IDictionary<,> with Dictionary<,> (doesn't actually modify tier1 JIT code, but make JIT's job easier opening potential future optimization)