Remove SILBasicBlock::getBBArgIndex(SILArgument *) in favor of SILArgument::getIndex().

The index is a property of the argument. There is no reason from a modeling
perspective to go through the BB to get it.

Swift SVN r21338

include/swift/SIL/SILBasicBlock.h

@@ -135,7 +135,6 @@ public:
   const SILArgument *getBBArg(unsigned i) const { return BBArgList[i]; }
   SILArgument *getBBArg(unsigned i) { return BBArgList[i]; }
   SILArgument *replaceBBArg(unsigned i, SILType Ty, ValueDecl *D =nullptr);
-  size_t getBBArgIndex(SILArgument *Arg);
 
   /// \brief Remove all block arguments.
   void dropAllArgs() { BBArgList.clear(); }