Small fixes to eliminate compiler warnings when using clang

llvmutil.cpp

@@ -549,7 +549,7 @@ lValuesAreEqual(llvm::Value *v0, llvm::Value *v1,
     llvm::CastInst *cast1 = llvm::dyn_cast<llvm::CastInst>(v1);
     if (cast0 != NULL && cast1 != NULL) {
         if (cast0->getOpcode() != cast1->getOpcode())
-            return NULL;
+            return false;
         return lValuesAreEqual(cast0->getOperand(0), cast1->getOperand(0),
                                seenPhi0, seenPhi1);
     }

opt.cpp

@@ -342,7 +342,7 @@ lGetMask(llvm::Value *factor, uint64_t *mask) {
             llvm::Constant *c = 
                 llvm::dyn_cast<llvm::Constant>(cv->getOperand(i));
             if (c == NULL)
-                return NULL;
+                return false;
             elements.push_back(c);
         }
 #else

stmt.cpp

@@ -2315,14 +2315,14 @@ ForeachUniqueStmt::TypeCheck() {
         Error(expr->pos, "Iteration domain type in \"foreach_tiled\" loop "
               "must be \"varying\" type, not \"%s\".",
               type->GetString().c_str());
-        return false;
+        return NULL;
     }
 
     if (Type::IsBasicType(type) == false) {
         Error(expr->pos, "Iteration domain type in \"foreach_tiled\" loop "
               "must be an atomic, pointer, or enum type, not \"%s\".",
               type->GetString().c_str());
-        return false;
+        return NULL;
     }
 
     return this;