lj_obj.h: Explicitly define implicit VM states

Define enums for implicit VM states that make sense even though they
are never explicitly stored in the g->vmstate field: jit head, jit
loop, jit garbage collection, and ffi.

src/lj_obj.h

@@ -438,13 +438,22 @@ typedef struct GCtab {
 
 /* VM states. */
 enum {
+  /* VM states. */
   LJ_VMST_INTERP,	/* Interpreter. */
   LJ_VMST_C,		/* C function. */
   LJ_VMST_GC,		/* Garbage collector. */
   LJ_VMST_EXIT,		/* Trace exit handler. */
   LJ_VMST_RECORD,	/* Trace recorder. */
   LJ_VMST_OPT,		/* Optimizer. */
   LJ_VMST_ASM,		/* Assembler. */
+  /* JIT trace states.
+  ** These are "abstract" states that logically exist but are never
+  ** directly used for the value of global_State.vmstate.
+  */
+  LJ_VMST_HEAD,		/* Trace mcode before loop */
+  LJ_VMST_LOOP,		/* Trace mcode inside loop */
+  LJ_VMST_JGC,		/* GC invoked from JIT mcode. */
+  LJ_VMST_FFI,		/* Other code outside trace mcode */
   LJ_VMST__MAX
 };