AMDGPU: Error more gracefully on libcalls

I think this is the only situation where the callsite
will have a null instruction.

llvm-svn: 339271
diff --git a/llvm/lib/Target/AMDGPU/SIISelLowering.cpp b/llvm/lib/Target/AMDGPU/SIISelLowering.cpp
index b763685..439006e 100644
--- a/llvm/lib/Target/AMDGPU/SIISelLowering.cpp
+++ b/llvm/lib/Target/AMDGPU/SIISelLowering.cpp
@@ -2376,6 +2376,9 @@
                               "unsupported call to variadic function ");
   }
 
+  if (!CLI.CS.getInstruction())
+    report_fatal_error("unsupported libcall legalization");
+
   if (!CLI.CS.getCalledFunction()) {
     return lowerUnhandledCall(CLI, InVals,
                               "unsupported indirect call to function ");