[Target/X86] Allow LowerVAARG to work on FreeBSD, too.

[cdavis5x]

Summary

This change enables the va_arg instruction to work on
FreeBSD/amd64, too. It's needed to prevent test failures after D1622
lands. Since FreeBSD/amd64 largely follows the System V ABI for x86-64
(like Linux/x86-64), I don't see any reason why this wouldn't work.

[dim]

LGTM.

[cdavis5x]

Abandoning since I gutted the assert(3) in D1622.

lib/Target/X86/X86ISelLowering.cpp

SDValue X86TargetLowering::LowerVASTART(SDValue Op, SelectionDAG &DAG) const {
   MemOps.push_back(Store);
   return DAG.getNode(ISD::TokenFactor, DL, MVT::Other, MemOps);
 }
 
 SDValue X86TargetLowering::LowerVAARG(SDValue Op, SelectionDAG &DAG) const {
   assert(Subtarget->is64Bit() &&
          "LowerVAARG only handles 64-bit va_arg!");
   assert((Subtarget->isTargetLinux() ||
+          Subtarget->isTargetFreeBSD() ||
           Subtarget->isTargetDarwin()) &&
           "Unhandled target in LowerVAARG");
   assert(Op.getNode()->getNumOperands() == 4);
   SDValue Chain = Op.getOperand(0);
   SDValue SrcPtr = Op.getOperand(1);
   const Value *SV = cast<SrcValueSDNode>(Op.getOperand(2))->getValue();
   unsigned Align = Op.getConstantOperandVal(3);
   SDLoc dl(Op);