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

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

> LG. I can successfully build the patch on AIX without encountering the previously described error.

@kkwli thanks for checking, I updated the code to just use LLVM_PACKED_START/END, so this is now done unconditionally

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