[llvm] IRLinker: avoid quadratic behavior (PR #157045)

[Hans Wennborg via llvm-commits]

zmodem wrote:

We're hitting the assert when there are duplicated `!llvm.linker.options` in the destination module. (https://crbug.com/443585789)

I'll go ahead and remove the assert for now and add a test.

https://github.com/llvm/llvm-project/pull/157045