swift234

SILGen: Unwind some levels of abstraction in argument binding.

Have the ArgumentInitVisitor directly bind argument variables to BB arguments, instead of trying to reuse the InitializationForPattern logic used for general variable bindings. That was a nice idea, but it leads to some ugly edge cases because of the many little ways arguments are different from local variable bindings. By getting rid of the abstraction layers, it's easy for argument binding to bind +0 guaranteed or +1 arguments in place when appropriate, avoiding an r/r pair for "let" bindings. It will also let us eliminate some ugly code from variable binding initialization. Should be NFC aside from some harmless reordering of prolog/epilog variable setup.

Swift SVN r24412