Undo unsound change to getValue

[wmdietl]

When boxing primitive values, an artificial call like Integer.valueOf(arg) is created in the CFG.
The tree arg then maps to the post-conversion node.
The first iteration to the CFG will not find a value for arg and the previous change to getValue then uses the pre-conversion node instead.

In the test case, this only happens in simple CFGs and switching pre- and post-conversion nodes has no impact.
However, if a fix-point iteration is necessary, this can lead to using the post-conversion value as argument to valueOf, which is wrong.

This PR introduces a new temporary variable to hold the argument to valueOf, to ensure they are not mixed up.
However, this has a large performance overhead: checker/tests/all-systems/Issue301.java on my system now takes more than double the time to check, with both the Nullness and Signedness Checkers.

Items:

• Write a test type system where Integer.valueOf takes one type and returns an incompatible different type. Write a test case that requires a fix-point iteration and see whether this exposes the problem with the original code.
• See whether there is a more efficient way to achieve sound behavior
• The code in CFGTranslationPhaseOne is very similar to other code for new temporary variables. See how to extract this into a helper function.