Fix two issues in `MatchingSubsets::populateSubsetOpsAtIterArg`:
1. The `collectHoistableOps` parameter was declared but never used when
inserting subset ops via `insert(subsetOp)`. As a result, when recursing
into nested loops with `collectHoistableOps=false`, the nested loop's
subset ops were incorrectly added to the hoistable extraction/insertion
pairs of the parent loop. This caused spurious failures in the
`allDisjoint` check, preventing valid hoisting when nested loop ops
overlapped with outer loop ops. Fix by passing the parameter:
`insert(subsetOp, collectHoistableOps)`.
2. In the nested loop handling branch, there was no guard to detect when
a value has multiple nested loop uses (i.e., is used as an init arg in
more than one nested loop). Without the guard, `nextValue` would be
silently overwritten, leading to an incorrect use-def chain traversal.
Add `if (nextValue) return failure()` before setting `nextValue` for the
nested loop case, mirroring the existing guard for insertion ops.
Fixes#147096
Assisted-by: Claude Code
5d293008c2