[llvm] [LoongArch] Fix MergeBaseOffset for constant pool index operand (PR #159336)

[via llvm-commits]

heiher wrote:

> LGTM. And is it necessary to cherry-pick to release/21.x?

Thanks. It should be cherry-picked to release/21.x.

https://github.com/llvm/llvm-project/pull/159336