[llvm] [GlobalMerge]Prefer use global-merge-max-offset instead of the target-specific constant offset. (PR #165591)

[via llvm-commits]

hstk30-hw wrote:

> Actually, could we delete this code too? I don't think it is ever used.
> 
> ```
>   explicit GlobalMerge() : FunctionPass(ID) {
>     Opt.MaxOffset = GlobalMergeMaxOffset;
>     Opt.MergeConstantGlobals = EnableGlobalMergeOnConst;
>     Opt.MergeConstAggressive = GlobalMergeAllConst;
>     initializeGlobalMergePass(*PassRegistry::getPassRegistry());
>   }
> ```

This is for `opt` tool, delete it will let `opt -global-merge` fail.

https://github.com/llvm/llvm-project/pull/165591