middle-end: Support early break/return auto-vectorization.

[juzhe.zh...@rivai.ai]

Hi, this patch is very interesting patch and I found it's very beneficial after 
applying to my downstream RVV GCC.
However, it has been a long time that this patch didn't update.
Is it possible that this patch will be refined and merged into trunk in the 
future ?

Thanks


juzhe.zh...@rivai.ai