[Transforms] classifyArgUse - don't deference pointer before null test

Reported here: https://pvs-studio.com/en/blog/posts/cpp/1003/ (N11)

Author: RKSimon

llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp

@@ -2621,16 +2621,15 @@ void LibCallSimplifier::classifyArgUse(
     SmallVectorImpl<CallInst *> &SinCalls,
     SmallVectorImpl<CallInst *> &CosCalls,
     SmallVectorImpl<CallInst *> &SinCosCalls) {
-  CallInst *CI = dyn_cast<CallInst>(Val);
-  Module *M = CI->getModule();
-
+  auto *CI = dyn_cast<CallInst>(Val);
   if (!CI || CI->use_empty())
     return;
 
   // Don't consider calls in other functions.
   if (CI->getFunction() != F)
     return;
 
+  Module *M = CI->getModule();
   Function *Callee = CI->getCalledFunction();
   LibFunc Func;
   if (!Callee || !TLI->getLibFunc(*Callee, Func) ||