[llvm] [VPlan] Use pragma pack(1) for VPIRFlags on AIX. (PR #184687)

[Florian Hahn via llvm-commits]

fhahn wrote:

@kkwli @hubert-reinterpretcast I think this should fix the build failure mentioned in https://github.com/llvm/llvm-project/pull/181571 for AIX, but I am not able to verify locally. Would be great if you could help

https://github.com/llvm/llvm-project/pull/184687