[llvm] [DebugInfo][LICM] Fix missing debug location updates (PR #91729)

[Shan Huang via llvm-commits]

Apochens wrote:

> It would be an unusual use of `assert` for sure, and I think it would be preferable to have a test that checks the location is lost. Then the test is already written, it will fail when we handle line locations differently, and a comment in the test can say we expect this to need updating when we fix hoist to do the right thing.

Actually, the `fdiv` with debug location dropped can be tested by the same test case (ie., `preserving-debugloc-fmul.ll`). I can add checks for that in this test and a comment pointing out this special case. Would that be acceptable?

https://github.com/llvm/llvm-project/pull/91729