[Polly] [PPCGCodeGeneration] Mild refactoring of checking validity of functions in a kernel.

[bollu]

Summary

This is a stylistic change to make the function a little more readable.
Also add a debug print to show what instruction contains a use of a
function we don't understand in the kernel.

[grosser]

LGTM

polly/trunk/lib/CodeGen/PPCGCodeGeneration.cpp

public:
   ///
   /// If this basic block does something with a `Function` other than calling
   /// a function that we support in a kernel, return true.
   bool containsInvalidKernelFunctionInBlock(const BasicBlock *BB,
                                             bool AllowCUDALibDevice) {
     for (const Instruction &Inst : *BB) {
       const CallInst *Call = dyn_cast<CallInst>(&Inst);
       if (Call && isValidFunctionInKernel(Call->getCalledFunction(),
-                                          AllowCUDALibDevice)) {
+                                          AllowCUDALibDevice))
         continue;
-      }
 
-      for (Value *SrcVal : Inst.operands()) {
-        PointerType *p = dyn_cast<PointerType>(SrcVal->getType());
-        if (!p)
-          continue;
-        if (isa<FunctionType>(p->getElementType()))
+      for (Value *Op : Inst.operands())
+        // Look for (<func-type>*) among operands of Inst
+        if (auto PtrTy = dyn_cast<PointerType>(Op->getType())) {
+          if (isa<FunctionType>(PtrTy->getElementType())) {
+            DEBUG(dbgs() << Inst
+                         << " has illegal use of function in kernel.\n");
             return true;
           }
         }
+    }
     return false;
   }
 
   /// Return whether the Scop S uses functions in a way that we do not support.
   bool containsInvalidKernelFunction(const Scop &S, bool AllowCUDALibDevice) {
     for (auto &Stmt : S) {
       if (Stmt.isBlockStmt()) {
         if (containsInvalidKernelFunctionInBlock(Stmt.getBasicBlock(),