Collection expression span optimization should reuse temps when possible

[RikkiGibson]

Collection expression span optimization should reuse temps when possible

[jaredpar]

The case I'm most concerned about is the params case. It's common to call params methods in groups for example if you're using Console.WriteLine or StringBuilder.AppendFormat it a series of statements. That is a place where the temp locals could build up and a better re-use plan would be valuable.

I also think that scoping the optimization to that makes it more approachable. The advantage of params methods is it's very simple to detect if the collection expression is inaccessible after the method returns. Methods that meet the following criteria can be assured the generated collection expression value cannot live beyond the method invocation:

1. The type of params is a ref struct
2. The return type of the method is not a ref struct
3. There are no parameters that are in / out / ref with a type of a ref struct