Refactor top-level comma rewriting.

Refactor top-level comma rewriting.

Top-level commas were being handled in three separate parts of
rationalize: TreeTransformRationalization, RewriteSimpleTransforms,
and DoPhase. Furthermore, the transform performed by the latter
was distinct from the transforms performed by the former two parts,
as those parts called out to RewriteTopLevelComma in order to do
the necessary rewrite. This change collects the two transforms
in RewriteTopLevelComma and centralizes the comma handling in
RewriteSimpleTransforms.

With the comma handling centralized in RewriteSimpleTransforms,
TreeTransformRationalization was not doing much of anything, so
it has been removed and RewriteSimpleTransforms renamed to
TreeTransformRationalization.