[llvm] Enable aggressive constant merge in GlobalMerge for AIX (PR #113956)

[Zaara Syeda via llvm-commits]

syzaara wrote:

> @syzaara your change is causing the test CodeGen/PowerPC/gcov_ctr_ref_init.ll to fail on several bots. Can you take a look and revert if you need time to investigate?

I merged a follow-up fix to the test: https://github.com/llvm/llvm-project/pull/114428

https://github.com/llvm/llvm-project/pull/113956