patch 8.2.1390: Vim9: type error after storing an option value

Problem:    Vim9: type error after storing an option value.
Solution:   Drop the type after a STOREOPT instruction. (closes #6632)
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 570885a..a6b4a27 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -1336,7 +1336,7 @@
     isn_T	*isn;
 
     RETURN_OK_IF_SKIP(cctx);
-    if ((isn = generate_instr(cctx, ISN_STOREOPT)) == NULL)
+    if ((isn = generate_instr_drop(cctx, ISN_STOREOPT, 1)) == NULL)
 	return FAIL;
     isn->isn_arg.storeopt.so_name = vim_strsave(name);
     isn->isn_arg.storeopt.so_flags = opt_flags;