[clang] [clang][AArch64] Avoid a crash when a non-reserved register is used (PR #117419)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

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

Full details are available at: https://lab.llvm.org/buildbot/#/builders/144/builds/13353

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

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'Clang :: CodeGen/AArch64/fixed-register-global.c' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
RUN: at line 4: /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/clang -c --target=aarch64-none-gnu -ffixed-x15 /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/CodeGen/AArch64/fixed-register-global.c 2>&1 | /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/count 0
+ /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/clang -c --target=aarch64-none-gnu -ffixed-x15 /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/CodeGen/AArch64/fixed-register-global.c
+ /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/count 0
Expected 0 lines, got 2.

--

********************


```

</details>

https://github.com/llvm/llvm-project/pull/117419