AMDGPU: Fix UBSan error caused by r335942

Summary: Fixes PR38071.

Reviewers: arsenm, dstenb

Reviewed By: arsenm

Subscribers: kzhuravl, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D48979

llvm-svn: 336448
diff --git a/llvm/lib/Target/AMDGPU/R600ISelLowering.cpp b/llvm/lib/Target/AMDGPU/R600ISelLowering.cpp
index 0c31deb..bddab43 100644
--- a/llvm/lib/Target/AMDGPU/R600ISelLowering.cpp
+++ b/llvm/lib/Target/AMDGPU/R600ISelLowering.cpp
@@ -1658,7 +1658,7 @@
 
     unsigned ValBase = ArgLocs[In.getOrigArgIndex()].getLocMemOffset();
     unsigned PartOffset = VA.getLocMemOffset();
-    unsigned Offset = Subtarget->getExplicitKernelArgOffset(MF) +
+    unsigned Offset = Subtarget->getExplicitKernelArgOffset(MF.getFunction()) +
                       VA.getLocMemOffset();
 
     MachinePointerInfo PtrInfo(UndefValue::get(PtrTy), PartOffset - ValBase);