[llvm] IRBuilder: avoid crash when seeking to start of a BasicBlock with only DebugInfo (PR #66266)

[Augie Fackler via llvm-commits]

durin42 wrote:

@jmorse rebased and rewritten - but I still can't return `DebugInfo()` in the case when `getNextNonDebugInstruction()` returns null. I've folded in the unit test, and I think cleaned up the style. Please take a look and let me know what you think?

Thanks!

https://github.com/llvm/llvm-project/pull/66266