[llvm] [InstCombine] Retain debug information on store to null instruction (PR #86105)

[Felipe de Azevedo Piovezan via llvm-commits]

felipepiovezan wrote:

> @OCHyams Thanks for the reduced test case. @dwblaikie Can you please let us know if this is enough coverage? If yes, can we merge it ?

Please update the PR so we can see what the final test will look like, with the relevant `CHECK` lines :)
But yeah, in general the test above is much better than the original one

https://github.com/llvm/llvm-project/pull/86105