[Bug tree-optimization/96053] Miss optimization:Finding SLP sequences from reductions sometimes is better than finding from reduction chains

[zhoukaipeng3 at huawei dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96053

--- Comment #4 from Kaipeng Zhou <zhoukaipeng3 at huawei dot com> ---
This patch add #pragma GCC no_reduc_chain and only completes the front end of C
language.

For the testcase, it successfully skipped doing slp by finding sequences from
reduction chains.  Without #pragma GCC no_reduc_chain, it will fail to do
vectorization.

Please help to check if there is any problem. If there is no problem, I will
continue to complete the front end of the remaining languages.