[llvm] [llvm-reduce] Fix incorrectly ignored null MD in ReduceDIMetadata (PR #108541)

[Stephen Tozer via llvm-commits]

SLTozer wrote:

I'll look for a test case when I'm able, but if you've triggered this bug recently and have a reproducer to-hand (reduced preferably but still usable otherwise) that would be helpful - if I recall, I tried to reduce a testcase at the time I fixed this, but it proved more finnicky than I expected to reduce a test case that invokes llvm-reduce, so I set it aside and have since lost my original reproducer.

https://github.com/llvm/llvm-project/pull/108541