swift234

Move to use SSAUpdater to generate the SILArgument when a location has a covering value,
i.e. multiple different values from predecessors

Previously, RLE is placing the SILArguments and branch edgevalues itself. This is probably
not as reliable/robust as using the SSAupdater.

RLE uses a single SSAupdater to create the SILArguments, this way previously created SILArguments
can be reused.

One test is created specifically for that so that we do not generate extraneous SILArguments.