Adding comments (dotnet/coreclr#23327)

Adding comments (dotnet/coreclr#23327)

* Adding comments to psiScope functions declarations

* Adding comments on declaration/definition Compiler::compUpdateLife

* Editing CodeGen::genProduceReg header comments

* Editting var Compiler::lvaCount comment

* Adding comments to Compiler::compChangeLife function

* Adding comments when changing variable liveness

* Adding comments in genSetScopeInfo()

Commit migrated from https://github.com/dotnet/coreclr/commit/728740987f4afcb8f55fe89a309471f01c62ef06