[llvm] [AMDGPU] Use ListSeparator. NFC. (PR #169347)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `ppc64le-mlir-rhel-clang` running on `ppc64le-mlir-rhel-test` while building `llvm` at step 6 "test-build-check-mlir-build-only-check-mlir".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/129/builds/33767

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-check-mlir-build-only-check-mlir) failure: 1200 seconds without output running [b'ninja', b'check-mlir'], attempting to kill
...
PASS: MLIR-Unit :: IR/./MLIRIRTests/0/130 (3670 of 3681)
PASS: MLIR-Unit :: IR/./MLIRIRTests/101/130 (3671 of 3681)
PASS: MLIR-Unit :: IR/./MLIRIRTests/100/130 (3672 of 3681)
PASS: MLIR-Unit :: IR/./MLIRIRTests/39/130 (3673 of 3681)
PASS: MLIR-Unit :: IR/./MLIRIRTests/38/130 (3674 of 3681)
PASS: MLIR-Unit :: Interfaces/./MLIRInterfacesTests/11/22 (3675 of 3681)
PASS: MLIR-Unit :: Interfaces/./MLIRInterfacesTests/13/22 (3676 of 3681)
PASS: MLIR-Unit :: Pass/./MLIRPassTests/10/13 (3677 of 3681)
PASS: MLIR-Unit :: Interfaces/./MLIRInterfacesTests/12/22 (3678 of 3681)
PASS: MLIR :: mlir-reduce/dce-test.mlir (3679 of 3681)
command timed out: 1200 seconds without output running [b'ninja', b'check-mlir'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=2191.157183

```

</details>

https://github.com/llvm/llvm-project/pull/169347