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

[Nikita Popov via llvm-commits]

nikic wrote:

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

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