[gcc r13-4434] rs6000: Corrects comments which are added by r13-4423

[gcc r13-4434] rs6000: Corrects comments which are added by r13-4423

[HaoChen Gui]

https://gcc.gnu.org/g:125f294e851b2282ee3bc88a6e5957cc90f013bd

commit r13-4434-g125f294e851b2282ee3bc88a6e5957cc90f013bd
Author: Haochen Gui <guihaoc@gcc.gnu.org>
Date:   Thu Dec 1 13:55:46 2022 +0800

    rs6000: Corrects comments which are added by r13-4423
    
    gcc/
            * config/rs6000/rs6000-call.cc (swap_endian_selector_for_mode):
            Corrects comments of this function and make them clear.

Diff:
---
 gcc/config/rs6000/rs6000-call.cc | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/gcc/config/rs6000/rs6000-call.cc b/gcc/config/rs6000/rs6000-call.cc
index c2a4e4f4e27..59c51fa3579 100644
--- a/gcc/config/rs6000/rs6000-call.cc
+++ b/gcc/config/rs6000/rs6000-call.cc
@@ -2802,8 +2802,10 @@ rs6000_gimplify_va_arg (tree valist, tree type, gimple_seq *pre_p,
   return build_va_arg_indirect_ref (addr);
 }
 
-/* The selector (perm) is expected to be used with vperm direct as the
-   function generates reversed perm for little endian with this patch.  */
+/* Return the permutation index for the swapping on the given vector mode.
+   Note that the permutation index is correspondingly generated by endianness,
+   it should be used by direct vector permutation.  */
+
 rtx
 swap_endian_selector_for_mode (machine_mode mode)
 {