[llvm] [PowerPC ][NFC] Add a test case for the function atomic_compare_exchange_weak (PR #141263)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `ml-opt-rel-x86-64` running on `ml-opt-rel-x86-64-b1` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/185/builds/19067

<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)
...
PASS: LLVM :: CodeGen/PowerPC/and-imm.ll (16987 of 58333)
PASS: LLVM :: CodeGen/PowerPC/aix_scalar_vector_permuted.ll (16988 of 58333)
PASS: LLVM :: CodeGen/PowerPC/and_sra.ll (16989 of 58333)
PASS: LLVM :: CodeGen/PowerPC/andc.ll (16990 of 58333)
PASS: LLVM :: CodeGen/PowerPC/anyext_srl.ll (16991 of 58333)
PASS: LLVM :: CodeGen/PowerPC/anon_aggr.ll (16992 of 58333)
PASS: LLVM :: CodeGen/PowerPC/and-mask.ll (16993 of 58333)
PASS: LLVM :: CodeGen/PowerPC/and_add.ll (16994 of 58333)
PASS: LLVM :: CodeGen/PowerPC/arr-fp-arg-no-copy.ll (16995 of 58333)
UNRESOLVED: LLVM :: CodeGen/PowerPC/atomic-compare-exchange-weak.ll (16996 of 58333)
******************** TEST 'LLVM :: CodeGen/PowerPC/atomic-compare-exchange-weak.ll' FAILED ********************
Test has unterminated 'RUN:' directive (with '\') from line 7 to 9
********************
PASS: LLVM :: CodeGen/PowerPC/asm-constraints.ll (16997 of 58333)
PASS: LLVM :: CodeGen/PowerPC/asm-Zy.ll (16998 of 58333)
PASS: LLVM :: CodeGen/PowerPC/arg_promotion.ll (16999 of 58333)
PASS: LLVM :: CodeGen/PowerPC/ashr-neg1.ll (17000 of 58333)
PASS: LLVM :: CodeGen/PowerPC/asm-printer-topological-order.ll (17001 of 58333)
PASS: LLVM :: CodeGen/PowerPC/alloca-crspill.ll (17002 of 58333)
PASS: LLVM :: CodeGen/PowerPC/asym-regclass-copy.ll (17003 of 58333)
PASS: LLVM :: CodeGen/PowerPC/asm-template-I.ll (17004 of 58333)
PASS: LLVM :: CodeGen/PowerPC/and_sext.ll (17005 of 58333)
PASS: LLVM :: CodeGen/PowerPC/atomic-1.ll (17006 of 58333)
PASS: LLVM :: CodeGen/PowerPC/atomics-constant.ll (17007 of 58333)
PASS: LLVM :: CodeGen/PowerPC/atomicrmw-cond-sub-clamp.ll (17008 of 58333)
PASS: LLVM :: CodeGen/PowerPC/atomicrmw-uinc-udec-wrap.ll (17009 of 58333)
PASS: LLVM :: CodeGen/PowerPC/aix-overflow-toc-data.py (17010 of 58333)
PASS: LLVM :: CodeGen/PowerPC/atomic-float.ll (17011 of 58333)
PASS: LLVM :: CodeGen/PowerPC/asm-dialect.ll (17012 of 58333)
PASS: LLVM :: CodeGen/PowerPC/atomic-minmax.ll (17013 of 58333)
PASS: LLVM :: CodeGen/PowerPC/atomics-fences.ll (17014 of 58333)
PASS: LLVM :: CodeGen/PowerPC/atomics-indexed.ll (17015 of 58333)
PASS: LLVM :: CodeGen/PowerPC/basic-toc-data-def.ll (17016 of 58333)
PASS: LLVM :: CodeGen/PowerPC/atomic-2.ll (17017 of 58333)
PASS: LLVM :: CodeGen/PowerPC/basic-toc-data-local-linkage.ll (17018 of 58333)
PASS: LLVM :: CodeGen/PowerPC/basic-toc-data-private-linkage.ll (17019 of 58333)
PASS: LLVM :: CodeGen/PowerPC/available-externally.ll (17020 of 58333)
PASS: LLVM :: CodeGen/PowerPC/atomics.ll (17021 of 58333)
PASS: LLVM :: CodeGen/PowerPC/basic-toc-data-extern.ll (17022 of 58333)
PASS: LLVM :: CodeGen/PowerPC/bdzlr.ll (17023 of 58333)
PASS: LLVM :: CodeGen/PowerPC/big-endian-formal-args.ll (17024 of 58333)
PASS: LLVM :: CodeGen/PowerPC/big-endian-actual-args.ll (17025 of 58333)
PASS: LLVM :: CodeGen/PowerPC/big-endian-store-forward.ll (17026 of 58333)
PASS: LLVM :: CodeGen/PowerPC/atomics-i128-ldst.ll (17027 of 58333)
PASS: LLVM :: CodeGen/PowerPC/big-endian-call-result.ll (17028 of 58333)
PASS: LLVM :: CodeGen/PowerPC/bcd-intrinsics.ll (17029 of 58333)
PASS: LLVM :: CodeGen/PowerPC/bitcast-peephole.mir (17030 of 58333)
PASS: LLVM :: CodeGen/PowerPC/block-placement-1.mir (17031 of 58333)
PASS: LLVM :: CodeGen/PowerPC/bfloat16-outer-product.ll (17032 of 58333)

```

</details>

https://github.com/llvm/llvm-project/pull/141263