llvm/ObjCARC: Split OptimizeIndividualCallImpl out of OptimizeIndivid…

…ualCalls, NFC

Split out a helper function for the individual call optimizations and
skip useless calls to it (where the instruction is not an ARC
intrinsic).  Besides reducing indentation (and possibly speeding up
compile time in some small way), an upcoming patch will add additional
calls and expand out the `switch`.