The revision distinguishes `ReduceFn` and `ReduceFnUse`. The latter has the reduction dimensions attached while the former specifies the arithmetic function only. This separation allows us to adapt the reduction syntax a little bit and specify the reduction dimensions using square brackets (in contrast to the round brackets used for the values to reduce). It als is a preparation to add reduction function attributes to OpDSL. A reduction function attribute shall only specify the arithmetic function and not the reduction dimensions.
Example:
```
ReduceFn.max_unsigned(D.kh, D.kw)(...)
```
changes to:
```
ReduceFn.max_unsigned[D.kh, D.kw](...)
```
Depends On D115240
Reviewed By: stellaraccident
Differential Revision: https://reviews.llvm.org/D115241
e3b442b62f