swift234

[SIL] Propagate basic block arguments.

Propagate/remove basic block input values when all predecessors supply
the same arguments. Another optimization needed to fold 'true'.

TODO: I need to figure out additional surcumstances in which to to trigger this
optimization and if we need to rerun CCP after this. (For example, we need to
rerun this after constant folding the terminator in order to fold 'true'.)

Swift SVN r8518