patch 8.2.2842: Vim9: skip argument to searchpair() is not compiled

Problem:    Vim9: skip argument to searchpair() is not compiled.
Solution:   Add VAR_INSTR.
diff --git a/src/evalvars.c b/src/evalvars.c
index 1b0b7d0..f3b4da2 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -1912,6 +1912,7 @@
 	case VAR_SPECIAL:
 	case VAR_JOB:
 	case VAR_CHANNEL:
+	case VAR_INSTR:
 	    break;
 
 	case VAR_BLOB: