Move promote methods to TIntermediate class

Move promote methods to TIntermediate class

A need arose to use capabilities from TIntermediate during
node promotion.  These methods have been moved from virtual
methods on the TIntermUnary and TIntermBinary nodes to methods
on TIntermediate, so it is easy for them construct new nodes
and so on.

This is done as a separate commit to verify that no test results
are changed as a result.