[llvm] AMDGPU: Use poison in some lowering contexts (PR #137325)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-x86_64-gcc-ubuntu` running on `sie-linux-worker3` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/174/builds/16774

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

```
Step 6 (test-build-unified-tree-check-all) failure: 1200 seconds without output running [b'ninja', b'check-all'], attempting to kill
...
PASS: lit :: shtest-not.py (90408 of 90418)
PASS: lit :: allow-retries.py (90409 of 90418)
PASS: lit :: discovery.py (90410 of 90418)
PASS: lit :: shtest-external-shell-kill.py (90411 of 90418)
PASS: lit :: googletest-timeout.py (90412 of 90418)
PASS: lit :: selecting.py (90413 of 90418)
PASS: lit :: shtest-timeout.py (90414 of 90418)
PASS: lit :: max-time.py (90415 of 90418)
PASS: lit :: shtest-shell.py (90416 of 90418)
PASS: lit :: shtest-define.py (90417 of 90418)
command timed out: 1200 seconds without output running [b'ninja', b'check-all'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1848.307992

```

</details>

https://github.com/llvm/llvm-project/pull/137325