GH-121784: Generate an error during code gen if a variable is marked `unused`, but...

GH-121784: Generate an error during code gen if a variable is marked `unused`, but is used and thus cached in a prior uop. (#121788)

* Reject uop definitions that declare values as 'unused' that are already cached by prior uops

* Track which variables are defined and only load from memory when needed

* Support explicit `flush` in macro definitions.

* Make sure stack is flushed in where needed.