More uniformity in code generation for 'for' loops

Added new instruction 'OP_TFORPREP' to prepare a generic for loop.
Currently it is equivalent to a jump (but with a format 'iABx',
similar to other for-loop preparing instructions), but soon it will
be the place to create upvalues for closing loop states.

lvm.c

@@ -1744,6 +1744,10 @@ void luaV_execute (lua_State *L, CallInfo *ci) {
         pc += GETARG_Bx(i);
         vmbreak;
       }
+      vmcase(OP_TFORPREP) {
+        pc += GETARG_Bx(i);
+        vmbreak;
+      }
       vmcase(OP_TFORCALL) {
         StkId cb = ra + 3;  /* call base */
         setobjs2s(L, cb+2, ra+2);