[llvm] [PPCMergeStringPool] Only replace constant once (PR #92996)

[Stefan Pintilie via llvm-commits]

stefanp-ibm wrote:

Thank you for adding a test case!
I have tried the test case but it seems to pass both with and without your patch. 
```
$ llvm-lit -v test/CodeGen/PowerPC/mergeable-string-pool-pr92991.ll
-- Testing: 1 tests, 1 workers --
PASS: LLVM :: CodeGen/PowerPC/mergeable-string-pool-pr92991.ll (1 of 1)

Testing Time: 0.07s

Total Discovered Tests: 1
  Passed: 1 (100.00%)
$ git log --oneline
f64732195c10 (HEAD) [FunctionAttrs] Regenerate test checks (NFC)
ca478bc6cc0e [SCEV] Support ule/sle exit counts via widening (#92206)
5ec91b392d7e [AttributorLight] Without liveness checks, look at all functions (#91004)
7f5c71efc441 InterleavedLoadCombine: Correctly query PM for TargetTransformInfo (#93103)
896bceb9537e [MC/DC][Coverage] Add assertions into emitSourceRegions() (#89572)
beac910c3bfc [nfc][InstrProfWriter]Wrap vtable writes in a method. (#93081)
2375921d6734 [ProfileData] Use default member initializations (NFC) (#93120)
aa4069ea96e5 Revert "[X86] Remove knl/knm specific ISAs supports (#92883)" (#93123)

```

It is possible that I have done something wrong here but all I did was rewind to before your change and then copy the test in. 

https://github.com/llvm/llvm-project/pull/92996