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

[Abinaya Saravanan via llvm-commits]

quic-asaravan wrote:

> This'll need test coverage (a fully reduced IR test case)
> 
> And it'd be worth having an explanation of what instructions are the input to this instruction (to know what location it should have) and whether they're in the same basic block or not (it's OK to preserve the location if we aren't making locations reachable that were not reachable in the original program): https://llvm.org/docs/HowToUpdateDebugInfo.html

I tried my best to reduce the test case. Find attached the lit test.
[test.txt](https://github.com/llvm/llvm-project/files/15008571/test.txt)


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