[llvm] [llvm] don't merge constants with COMDAT (PR #192477)

[via llvm-commits]

8051Enthusiast wrote:

I did now run some statistics on how the stage2 clang build is affected.
Without #190995 this doesn't change anything, but after reapplying it locally, the number of merged constants go down from 2699 to 2489 even when merging same comdats which is probably a bit too much.

So I added another criterion that merges constants when one doesn't have comdat and the other with comdat has local linkage which brought it back up to 2682 merges.

Also added some comments.

https://github.com/llvm/llvm-project/pull/192477