argdist expression support for USDT args

Currently, **argdist** doesn't support expressions over USDT arguments, e.g. `u::someprobe():int:arg3/1024`. Only plain argument expressions such as `arg3` are supported in that position. This should probably be fixed for consistency -- a simple approach would be to check which USDT argument indices appear in the expression, and then generate the appropriate `argN` variables and leave the original expression as is.

While we're at it, it might be a good idea to do this at once for all the expressions, because currently if e.g. `arg1` appears in more than one expression, we end up generating multiple local variables named `arg1`, which doesn't compile.

The relevant code is in `_generate_usdt_arg_assignment`.