[Bug target/112431] RISC-V GCC-15 feature: Support register overlap on widen RVV instructions

[juzhe.zhong at rivai dot ai]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112431

--- Comment #12 from JuzheZhong <juzhe.zhong at rivai dot ai> ---
Except vv/wv variant widen instructions.
All other widen EEW overlap have been done.

It seems that current register filter can not help us simulate accurate
highest-number overlap for vwadd.vv/vwadd.wv instructions.