Clean up struct allocation in gsParamsToShadows #13366

This method creates shadow copies (this is excerpted from the discussion here: dotnet/coreclr#21705 (comment)):

• first, it creates the variables, without calling lvaSetStruct, but copies some of the struct-related info, such as lvVerTypeInfo.
• then it changes the IR to use the new variables (ugly because the IR will now temporarily be in an invalid state - the newly created variables may have not been fully setup without lvaSetStruct)
• then it generates copies from the old variables to the new variables and here it calls lvaSetStruct

It should presumably call lvaSetStruct when it first creates the new locals.

category:implementation
theme:ir
skill-level:beginner
cost:small
impact:small