[IOperation] GetOperation returns a superset of IOperations for given syntax nodes

[333fred]

Uber-tracking issue for all instances of this. We seem to have a pattern of returning an IOperation that contains the expected operation for a given syntax node, rather than returning just the operation for that syntax node.

• [IOperation] Stop synthesizing ISwitchStatement when given a CaseSwitchLabel #18042 switch Statements
• [IOperation] Getting the variable declaration of a fixed statement returns entire fixed statement IOperation #18061 fixed Statements
• [IOperation] Getting the variable declaration of a using statement returns entire using statement IOperation #18062 using Statements
• [IOperation] GetOperation on an individual variable declaration in a multiple variable declaration synthesizes an unnecessary IVariableDeclarationStatement #18068 Variable declarations
• [IOperation] Getting the IOperation for a single argument returns the entire expression using the argument #18405 Argument expressions

[jinujoseph]

We dont' need the uber bug. All these are legitimate issues that just need to be fixed. Our design is that if you ask for a sub-syntax-node you should get the operation whose syntax points to that node. So all this existing behavior is definitely broken.