[Compiler] Optimize conversion between static and sema types

[RZhang05]

Avoid conversions where possible. Improve caching.

Possible approaches:

• Re-use static-type conversion results from compiler/ previous executions. See [Compiler] Re-use static-type to sema-type conversion results #4054 (comment)
• Avoid conversion from StaticType to sema.Type during runtime sub-type checking #3691
• Avoid static to sema type conversion when boxing (ConvertAndBox) [moved from https://github.com/Compiler Milestone X #3804]

[SupunS]

Related: #4054 (particularly #4054 (comment))