[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

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -668,6 +668,7 @@ public:
 };
 
 /// Class to record and manage LLVM IR flags.
+LLVM_PACKED_START
 class VPIRFlags {
   enum class OperationType : unsigned char {
     Cmp,
@@ -1070,6 +1071,7 @@ public:
   void printFlags(raw_ostream &O) const;
 #endif
 };
+LLVM_PACKED_END
 
 static_assert(sizeof(VPIRFlags) <= 3, "VPIRFlags should not grow");