[mlir:Async] Add the size parameter to the async.group
Specify the !async.group size (the number of tokens that will be added to it) at construction time. async.await_all operation can potentially race with async.execute operations that keep updating the group, for this reason it is required to know upfront how many tokens will be added to the group.
Reviewed By: ftynse, herhut
Differential Revision: https://reviews.llvm.org/D104780