The quick answer is "Yes" (wondering why you didnt just try it!)
SLL and SRL are functions, so all function parameters are constants. Constant function paramters can be connected to objects of any class (signal/variable/constant). Their values are copied into the function when it is called.
Dont expect good synth results if you try and do variable shifting though.
Oh sorry,
Forgot to mention - I did simulate it before posting...and it worked
What I really wanted to ask is about the synthesis support of tools - which you also answered...