[flang] Migrate away from PointerUnion::{is,get} (NFC) (#120880)

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>

I'm not touching PointerUnion::dyn_cast for now because it's a bit
complicated; we could blindly migrate it to dyn_cast_if_present, but
we should probably use dyn_cast when the operand is known to be
non-null.

flang/lib/Optimizer/CodeGen/CodeGen.cpp

@@ -1613,12 +1613,13 @@ struct EmboxCommonConversion : public fir::FIROpConversion<OP> {
         if (gepArgs.size() != 1)
           fir::emitFatalError(loc,
                               "corrupted substring GEP in fir.embox/fir.rebox");
-        mlir::Type outterOffsetTy = gepArgs[0].get<mlir::Value>().getType();
+        mlir::Type outterOffsetTy =
+            llvm::cast<mlir::Value>(gepArgs[0]).getType();
         mlir::Value cast =
             this->integerCast(loc, rewriter, outterOffsetTy, *substringOffset);
 
         gepArgs[0] = rewriter.create<mlir::LLVM::AddOp>(
-            loc, outterOffsetTy, gepArgs[0].get<mlir::Value>(), cast);
+            loc, outterOffsetTy, llvm::cast<mlir::Value>(gepArgs[0]), cast);
       }
     }
     mlir::Type llvmPtrTy = ::getLlvmPtrType(resultTy.getContext());