Add NumFixedArgs attribute to CallSDNode which indicates the number of fixed...

Add NumFixedArgs attribute to CallSDNode which indicates the number of fixed arguments in a vararg call.

With the SVR4 ABI on PowerPC, vector arguments for vararg calls are passed differently depending on whether they are a fixed or a variable argument. Variable vector arguments always go into memory, fixed vector arguments are put 
into vector registers. If there are no free vector registers available, fixed vector arguments are put on the stack.

The NumFixedArgs attribute allows to decide for an argument in a vararg call whether it belongs to the fixed or variable portion of the parameter list.

llvm-svn: 74764