Pirexamples

[sipma]

This pull request includes the following:

• initial redesign of handling stack variables. It replaces the classes StackLayout and FnStackFrame with the single class FunctionStackframe. The FunctionStackframe consists of a collection of Stackslots that form the logical division of a stackframe;
• improved handling of side-effect values, that is values that are assigned by a callee via a pointer passed to that callee;
• a collection of PIR examples that illustrate how predicated instructions are represented in PIR.