Improve testing of the codegen system

- Move codegen tests to a sub directory
- Add tests for all functions of Expression, ExpressionRef, and ExpressionGraph
- Respect dependencies during tests: The ExpressionGraph test doesn't
  use ExpressionRef anymore.

The new tests revealed a few bugs so the following changes were made:

- Expression::MakeNop now resets the current expression with the default
  constructed NOP expression
- ExpressionGraph::Insert now updates the lhs_id the same way as
  InsertBack()

Change-Id: I6a18925c1e4d972c29ec1219f2073b4eaf2df737