patch 9.0.0460: loop variable can't be found

Problem:    Loop variable can't be found.
Solution:   Adjust block_id of the loop variable each round.
diff --git a/src/vim.h b/src/vim.h
index 54a858a..8f1bf33 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -2258,6 +2258,7 @@
 #define ASSIGN_NO_MEMBER_TYPE 0x20 // use "any" for list and dict member type
 #define ASSIGN_FOR_LOOP 0x40 // assigning to loop variable
 #define ASSIGN_INIT	0x80 // not assigning a value, just a declaration
+#define ASSIGN_UPDATE_BLOCK_ID 0x100  // update sav_block_id
 
 #include "ex_cmds.h"	    // Ex command defines
 #include "spell.h"	    // spell checking stuff