Switch Alpha over to the new call lowering style. New code mostly

copied from the SystemZ target.  I don't think this causes any 
significant changes to the output (I compared the assembly, and the 
results appeared to be essentially unchanged), although I don't actually 
have an Alpha to test on.

I would appreciate if anyone with the appropriate hardware could test 
this. I'm not sure if that includes anyone subscribed to llvm-commits, 
though.

llvm-svn: 76353