[AbstractCallSite][FIX] Correct faulty assertion

When the Attributor run on the IPConstantProp test case for multiple
callbacks it triggered a faulty assertion in the AbstractCallSite
implementation. The callee can well be at argument position 0.

llvm/include/llvm/IR/CallSite.h

@@ -850,7 +850,7 @@ public:
   /// callee of this ACS. Only valid for callback calls!
   int getCallArgOperandNoForCallee() const {
     assert(isCallbackCall());
-    assert(CI.ParameterEncoding.size() && CI.ParameterEncoding[0] > 0);
+    assert(CI.ParameterEncoding.size() && CI.ParameterEncoding[0] >= 0);
     return CI.ParameterEncoding[0];
   }
 

llvm/test/Transforms/IPConstantProp/multiple_callbacks.ll

@@ -1,4 +1,5 @@
 ; RUN: opt -ipconstprop -S < %s | FileCheck %s
+; RUN: opt -S -passes=attributor -aa-pipeline='basic-aa' -attributor-disable=false -attributor-max-iterations-verify -attributor-max-iterations=1 < %s | FileCheck %s
 ;
 ;
 ;                            /---------------------------------------|