[Util] Ensure scf.index_switch ops share the same selector before merging (#25037)

`MergeIndexSwitchPattern` merges adjacent `scf.index_switch` operations
when their case values match, but it fails to verify that both ops
switch on the same condition operand (selector). Consequently, the
merged operation retains only the first op's selector, causing the
second op's regions to execute based on the wrong value.

### Minimal reproducer

Run with `iree-opt --iree-util-apply-patterns
--allow-unregistered-dialect`:

```mlir
util.func @f(%case0: index, %case1: index) -> (i32, i32) {
  %result0 = scf.index_switch %case0 -> i32
  case 0 { %v = "some.op0.case0"() : () -> i32
           scf.yield %v : i32 }
  case 1 { %v = "some.op0.case1"() : () -> i32
           scf.yield %v : i32 }
  default { %v = "some.op0.default"() : () -> i32
            scf.yield %v : i32 }
  %result1 = scf.index_switch %case1 -> i32
  case 0 { %v = "some.op1.case0"() : () -> i32
           scf.yield %v : i32 }
  case 1 { %v = "some.op1.case1"() : () -> i32
           scf.yield %v : i32 }
  default { %v = "some.op1.default"() : () -> i32
            scf.yield %v : i32 }
  util.return %result0, %result1 : i32, i32
}
```

Prior to this fix, the pattern collapses both into a single
`scf.index_switch %case0 -> i32, i32`, silently discarding `%case1`.

### Changes

- Require matching selector operands in `MergeIndexSwitchPattern` before
merging.
- Add a negative test case to
`Dialect/Util/Transforms/test/patterns.mlir` to ensure distinct
selectors prevent merging.

Assisted-by: Claude Code

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