Improve profile update in merge_blocks

[H.J. Lu]

Hi Jan,

I didn't find

https://gcc.gnu.org/cgit/gcc/commit/?id=8498ef3d075801

in

https://gcc.gnu.org/pipermail/gcc-patches/2025-October/date.html

In any case, your commit caused:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122122

-- 
H.J.