[SDAG] Remove FixedArgs parameter from CallLoweringInfo::setCallee

The setCallee function will set the number of fixed arguments based
on the size of the argument list. The FixedArgs parameter was often
explicitly set to 0, leading to a lack of consistent value for non-
vararg functions.

Differential Revision: http://reviews.llvm.org/D20376

llvm-svn: 273403