[PowerPC] Remove unnecessary load of r12 in indirect call

When looking at the 64-bit SVR4 indirect call sequence, I noticed
an unnecessary load of r12.  And indeed the code says:

  // R12 must contain the address of an indirect callee. 

But this is not correct; in the 64-bit SVR4 (ELFv1) ABI, there is
no need to load r12 at this point.  It seems this code and comment
is a remnant of code originally shared with the Darwin ABI ...

This patch simply removes the unnecessary load.

llvm-svn: 211203

llvm/lib/Target/PowerPC/PPCISelLowering.cpp

@@ -4399,10 +4399,6 @@ PPCTargetLowering::LowerCall_64SVR4(SDValue Chain, SDValue Callee,
     SDValue AddPtr = DAG.getNode(ISD::ADD, dl, PtrVT, StackPtr, PtrOff);
     Chain = DAG.getStore(Val.getValue(1), dl, Val, AddPtr, MachinePointerInfo(),
                          false, false, 0);
-    // R12 must contain the address of an indirect callee.  This does not
-    // mean the MTCTR instruction must use R12; it's easier to model this
-    // as an extra parameter, so do that.
-    RegsToPass.push_back(std::make_pair((unsigned)PPC::X12, Callee));
   }
 
   // Build a sequence of copy-to-reg nodes chained together with token chain