[Conversion] Migrate away from PointerUnion::{is,get} (NFC) (#122421)

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.

mlir/lib/Conversion/VectorToLLVM/ConvertVectorToLLVM.cpp

@@ -124,7 +124,7 @@ static Value getAsLLVMValue(OpBuilder &builder, Location loc,
     return builder.create<LLVM::ConstantOp>(loc, intAttr).getResult();
   }
 
-  return foldResult.get<Value>();
+  return cast<Value>(foldResult);
 }
 
 namespace {