Refactor and cleanup work for passing and returning struct types

Replaces argOrReturnTypeForStruct with two new methods:
  getArgTypeForStruct - Provides information on how to pass a struct type
  getReturnTypeForStruct - Provides information on how to return a struct type

A struct can be passed or return in the following different ways:
1. A struct type may be passed/returned as a primitive type when its size is small
2. A struct type may be passed/returned by reference to a copy
3. A struct type may be passed/returned by value using multiple registers
4. A struct type may be passed by value using a copy on the stack

Incorporated code review feedback with expanded comments.

Author: briansull