swift234

Add the method SILArgument::getIncomingValues and refactor SILArgument implementation from SILBasicBlock.cpp => SILArgument.cpp.

SILArgument::getIncomingValues() takes in an out array parameter and attempts to
gather up all values from the SILArguments parents predecessors whose value the
SILArgument could take on.

This will let me refactor the single predecessor handling code to also handle
multiple predecessors in a simple way.

Swift SVN r21864