[Util] Fix crash in MergeIndexSwitchPattern when second op yields outer values (#25038)

In `MergeIndexSwitchPattern`, remapping operands of the second
`scf.index_switch`'s yields used `IRMapping::lookupOrNull`. When a
yielded value was defined outside the switch region (e.g., hoisted
constants), it was absent from the mapper, resulting in a null operand
passed to `scf::YieldOp::create` and causing a crash.

Switch to `lookupOrDefault` for the second op's yielded values, matching
how the first op's yields are handled. Cloned values and results of the
first op are still remapped correctly, while values defined in enclosing
scopes pass through unchanged.

### Reproducer

Fails in `MergeIndexSwitchPattern::matchAndRewrite` →
`scf::YieldOp::create`:

```mlir
func.func @main(%sel: i32) -> (i32, i32) {
  %s = arith.index_cast %sel : i32 to index
  %a = scf.index_switch %s -> i32
  case 0 { %c = arith.constant 10 : i32
           scf.yield %c : i32 }
  case 1 { %c = arith.constant 11 : i32
           scf.yield %c : i32 }
  default { %c = arith.constant 12 : i32
            scf.yield %c : i32 }
  %b = scf.index_switch %s -> i32
  case 0 { %c = arith.constant 20 : i32
           scf.yield %c : i32 }
  case 1 { %c = arith.constant 21 : i32
           scf.yield %c : i32 }
  default { %c = arith.constant 22 : i32
            scf.yield %c : i32 }
  return %a, %b : i32, i32
}
```

```shell
iree-compile repro.mlir --iree-hal-target-device=local \
  --iree-hal-local-target-device-backends=vmvx -o /dev/null
```

### Changes

- Use `IRMapping::lookupOrDefault` when resolving yielded values from
the second switch.
- Add a regression test in `Dialect/Util/Transforms/test/patterns.mlir`
covering outer-scope yield values.

Assisted-by: Claude Code

Signed-off-by: jonathansong <7958352+jonathansong@users.noreply.github.com>
Co-authored-by: jonathansong <7958352+jonathansong@users.noreply.github.com>