[PowerPC] Do not merge TLS constants within PPCMergeStringPool.cpp (#94059)

This patch prevents thread-local constants to be merged within
PPCMergeStringPool.cpp.

The PPCMergeStringPool pass primarily merges non-thread-local constants
together, and thread-local constants should not be mixed together with
other (non-thread-local) constants. In the event that thread-local and
other non-thread-local constants are pooled together, the
llvm.threadlocal.address intrinsic can fail as it expects its argument
to be a thread-local global value, but the merged string structure
created by the PPCMergeStringPool pass is not thread-local as a whole.