[flang] Migrate away from PointerUnion::{is,get} (NFC)

Note that PointerUnion::{is,get} have been soft deprecated in
PointerUnion.h:

  // FIXME: Replace the uses of is(), get() and dyn_cast() with
  //        isa<T>, cast<T> and the llvm::dyn_cast<T>

Author: kazutakahirata

flang/include/flang/Optimizer/Dialect/CUF/CUFOps.td

@@ -222,7 +222,8 @@ def cuf_KernelLaunchOp : cuf_Op<"kernel_launch", [CallOpInterface,
     }
 
     void setCalleeFromCallable(mlir::CallInterfaceCallable callee) {
-      (*this)->setAttr(getCalleeAttrName(), callee.get<mlir::SymbolRefAttr>());
+      (*this)->setAttr(getCalleeAttrName(),
+                       llvm::cast<mlir::SymbolRefAttr>(callee));
     }
     mlir::FunctionType getFunctionType();
 

flang/include/flang/Optimizer/Dialect/FIROps.td

@@ -2488,8 +2488,9 @@ def fir_CallOp : fir_Op<"call",
     void setCalleeFromCallable(mlir::CallInterfaceCallable callee) {
       if (auto calling =
           (*this)->getAttrOfType<mlir::SymbolRefAttr>(getCalleeAttrName()))
-        (*this)->setAttr(getCalleeAttrName(), callee.get<mlir::SymbolRefAttr>());
-      setOperand(0, callee.get<mlir::Value>());
+        (*this)->setAttr(getCalleeAttrName(),
+                         llvm::cast<mlir::SymbolRefAttr>(callee));
+      setOperand(0, llvm::cast<mlir::Value>(callee));
     }
   }];
 }