[VPlan] Use pragma pack(1) for VPIRFlags on AIX. (#184687)
Some compilers (e.g. on AIX) do not pack by default. Use LLVM_PACKED to
ensure the VPIRFlags struct is packed as expected on all platforms.
This matches what we already do in other places for AIX, e.g. in
llvm/include/llvm/CodeGen/SelectionDAGNodes.h (added in
844a02e509a4cc03f76ef5dd1c358c57ee164b71), although it uses the more
general LLVM_PACKED unconditionally
PR: https://github.com/llvm/llvm-project/pull/184687
