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

[Stefan Pintilie via llvm-commits]

https://github.com/stefanp-ibm approved this pull request.

> @stefanp-ibm Does your LLVM build enable `-DLLVM_USE_SANITIZER=Address`? Alternatively, invoking opt under valgrind should also show the issue.

I was able to add `-DLLVM_USE_SANITIZER=Address` to the build and get this test to fail as expected. Thank you for fixing this!

LGTM.

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