Change InferType() to an abstract method and implement for every ILInstruction.
With this change, we now always have enough information to create a variable of an appropriate type to store the result of evaluating the instruction.
This previously was not the case for instructions producing "other value type", for which the stacktype-based fallback incorrectly produced `object`.
This initial commit only handles the trivial case where an Index or Range object is constructed.
The TODO portions of the test case show there are plenty of cases where where the C# compiler emits more complex code patterns that will require ILAst transforms.
This is a work-in-progress; ILSpy is not functional with this commit.
The old code path still exists but is broken because some classes were modified for the new system.
The new system is still highly incomplete (types only have fields, but no methods).