[llvm] [LoopIdiomVectorize] Recognize and transform minidx pattern (PR #144987)

[Sjoerd Meijer via llvm-commits]

sjoerdmeijer wrote:

> We can't block good improvements in earlier scalar passes based on the fact we no longer recognise the idiom here.

Agreed, of course. And this pattern matching needs to disappear anyway, long term. 

I guess the main thing left to assess and investigate whether this short term solution can be salvaged is what @fhahn wrote about legality:

> it looks like there are a number of legality checks missing, e.g. it doesn't check if there are any potential memory writes in the loop, and possibly misses checks if PRE is valid

The other comments look minor. 

https://github.com/llvm/llvm-project/pull/144987