[llvm] [InstSimplify] Fold `getelementptr inbounds null, idx -> null` (PR #130742)

[David Truby via llvm-commits]

DavidTruby wrote:

I didn't realise the recent xalancbmk failures we've been seeing are the same issue here (I hadn't got around to looking into that yet!)
Given the variety of failures I'd like to push a bit harder for a revert until fixes can be found for these failures

https://github.com/llvm/llvm-project/pull/130742