Fix compiliation of getmetatable() for UDTYPE_IO_FILE.

Reported by Sergey Bronnikov. #1279

src/lj_record.c

@@ -821,6 +821,7 @@ int lj_record_mm_lookup(jit_State *J, RecordIndex *ix, MMS mm)
   } else if (tref_isudata(ix->tab)) {
     int udtype = udataV(&ix->tabv)->udtype;
     mt = tabref(udataV(&ix->tabv)->metatable);
+    mix.tab = emitir(IRT(IR_FLOAD, IRT_TAB), ix->tab, IRFL_UDATA_META);
     /* The metatables of special userdata objects are treated as immutable. */
     if (udtype != UDTYPE_USERDATA) {
       cTValue *mo;
@@ -834,6 +835,8 @@ int lj_record_mm_lookup(jit_State *J, RecordIndex *ix, MMS mm)
       }
   immutable_mt:
       mo = lj_tab_getstr(mt, mmname_str(J2G(J), mm));
+      ix->mt = mix.tab;
+      ix->mtv = mt;
       if (!mo || tvisnil(mo))
 	return 0;  /* No metamethod. */
       /* Treat metamethod or index table as immutable, too. */
@@ -841,11 +844,8 @@ int lj_record_mm_lookup(jit_State *J, RecordIndex *ix, MMS mm)
 	lj_trace_err(J, LJ_TRERR_BADTYPE);
       copyTV(J->L, &ix->mobjv, mo);
       ix->mobj = lj_ir_kgc(J, gcV(mo), tvisfunc(mo) ? IRT_FUNC : IRT_TAB);
-      ix->mtv = mt;
-      ix->mt = TREF_NIL;  /* Dummy value for comparison semantics. */
       return 1;  /* Got metamethod or index table. */
     }
-    mix.tab = emitir(IRT(IR_FLOAD, IRT_TAB), ix->tab, IRFL_UDATA_META);
   } else {
     /* Specialize to base metatable. Must flush mcode in lua_setmetatable(). */
     mt = tabref(basemt_obj(J2G(J), &ix->tabv));