[Bug tree-optimization/79663] r244815 causes 10% regression for spec1k/172.mgrid on AArch64

[amker at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79663

--- Comment #2 from amker at gcc dot gnu.org ---
Unfortunately, 2) is not appropriate, in non-ZERO chain case, the root combined
statement is used to setup next iterations combined result, it's not for
CSE-use in this iteration.  Manually insert root_stmt at an earlier place could
result in def-after-use error.
Though for some cases, there is no def-after-use issue because all insertion
points are dominated by root_stmt.  This is not always the case.