Fix bootstrap failure (with g++ 4.8.5) in tree-if-conv.cc.

This patch fixes the bootstrap failure I'm seeing using gcc 4.8.5 as
the host compiler.

2023-07-17  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* tree-if-conv.cc (predicate_scalar_phi): Make the arguments
	to the std::sort comparison lambda function const.

gcc/tree-if-conv.cc

@@ -2204,7 +2204,8 @@ predicate_scalar_phi (gphi *phi, gimple_stmt_iterator *gsi)
     }
 
   /* Sort elements based on rankings ARGS.  */
-  std::sort(argsKV.begin(), argsKV.end(), [](ArgEntry &left, ArgEntry &right) {
+  std::sort(argsKV.begin(), argsKV.end(), [](const ArgEntry &left,
+					     const ArgEntry &right) {
     return left.second < right.second;
   });