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.

Author: brammool

src/testdir/test_vim9_expr.vim

@@ -43,6 +43,9 @@ def Test_expr1()
   var = 0
   assert_equal('two', var ? 'one' : 'two')
 
+  # with constant condition expression is not evaluated 
+  assert_equal('one', 1 ? 'one' : xxx)
+
   let Some: func = function('len')
   let Other: func = function('winnr')
   let Res: func = g:atrue ? Some : Other
@@ -139,7 +142,6 @@ enddef
 
 func Test_expr1_fails()
   call CheckDefFailure(["let x = 1 ? 'one'"], "Missing ':' after '?'", 1)
-  call CheckDefFailure(["let x = 1 ? 'one' : xxx"], "E1001:", 1)
 
   let msg = "white space required before and after '?'"
   call CheckDefFailure(["let x = 1? 'one' : 'two'"], msg, 1)

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1529,
 /**/
     1528,
 /**/

src/vim9compile.c

@@ -5519,6 +5519,7 @@ compile_elseif(char_u *arg, cctx_T *cctx)
     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_elseif(char_u *arg, cctx_T *cctx)
 
     // 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)