[OpenACC][NFC] Generalize wrapMultiBlockRegionWithSCFExecuteRegion (#187359)

Simplify `wrapMultiBlockRegionWithSCFExecuteRegion` by replacing the
`bool convertFuncReturn` parameter with a generic `getNumSuccessors() ==
0` check. Terminators with no successors are by definition region exit
points, so they can be identified automatically without requiring
callers to specify types. This enables downstream dialects (e.g., CUF
with fir::FirEndOp) to reuse the utility without modifying it.

```
// Before:
wrapMultiBlockRegionWithSCFExecuteRegion(region, mapping, loc, rewriter, /*convertFuncReturn=*/true);

// After:
wrapMultiBlockRegionWithSCFExecuteRegion(region, mapping, loc, rewriter);
```

GitOrigin-RevId: 593683f9a0eb53018ab1548ca55be9ff57cb1b23