patch 8.2.1529: Vim9: :elseif may be compiled when not needed

Problem:    Vim9: :elseif may be compiled when not needed.
Solution:   Do evaluate the :elseif expression.
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 3e9b42c..11a75c1 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -5519,6 +5519,7 @@
     isn_T	*isn;
     scope_T	*scope = cctx->ctx_scope;
     ppconst_T	ppconst;
+    skip_T	save_skip = cctx->ctx_skip;
 
     if (scope == NULL || scope->se_type != IF_SCOPE)
     {
@@ -5541,11 +5542,14 @@
 
     // compile "expr"; if we know it evaluates to FALSE skip the block
     CLEAR_FIELD(ppconst);
+    if (cctx->ctx_skip == SKIP_YES)
+	cctx->ctx_skip = SKIP_UNKNOWN;
     if (compile_expr1(&p, cctx, &ppconst) == FAIL)
     {
 	clear_ppconst(&ppconst);
 	return NULL;
     }
+    cctx->ctx_skip = save_skip;
     if (scope->se_skip_save == SKIP_YES)
 	clear_ppconst(&ppconst);
     else if (instr->ga_len == instr_count && ppconst.pp_used == 1)