Split parallel iteration space to simplify if statements

[ivanradanov]

The question still stands whether we want this enabled by default or only in some cases because it can have implications on code size.

[ivanradanov]

@wsmoses @ftynse

It looks like this results in less gathers and sliiightly less instructions overall???(or roughly the same)
this is after --affine-cfg --raise-affine-to-stablehlo on this IR

(ins)$ cat after-raise.split.mlir | grep gather | wc -l
144
(cmd)$ cat after-raise.mlir | grep gather | wc -l
186

(cmd)$ cat after-raise.split.mlir | wc -l
275455
(cmd)$ cat after-raise.mlir | wc -l
276531

This currently only handles 1d cases but may be worth to merge until we get 2d working