Drop bc field in GCproto since the bytecode is colocated.

src/lib_jit.c

@@ -210,7 +210,7 @@ LJLIB_CF(jit_util_funcbc)
   GCproto *pt = check_Lproto(L, 0);
   BCPos pc = (BCPos)lj_lib_checkint(L, 2) - 1;
   if (pc < pt->sizebc) {
-    BCIns ins = proto_ins(pt, pc);
+    BCIns ins = proto_bc(pt)[pc];
     BCOp op = bc_op(ins);
     lua_assert(op < BC__MAX);
     setintV(L->top, ins);