A SILArgument is a function argument if the argument's parent BB is the entry BB
of the function containing the argument.
This is an interesting distinction since function arguments have special
aliasing properties with respect to indirect arguments which normal basic block
arguments do not.
Swift SVN r14012
take a const ValueBase* instead of a SILValue, implement SILArgument
cases for a few visitors and opt others out explicitly, and assert
that classes in the SIL value hierarchy override their superclass's
classof.
Swift SVN r4705
