[llvm] [LV] Create in-loop sub reductions (PR #147026)

[David Green via llvm-commits]

davemgreen wrote:

The first of these appears to hit the assert about differences in the cost model. https://godbolt.org/z/GraEdK6Er
The second I'm not sure is gaining anything from performing the sub reducion in-loop with an extra vneg. It is more instructions in the loop, that is probably slower. It doesn't try to create an extending reduction though in this case, so isn't getting that wrong.

https://github.com/llvm/llvm-project/pull/147026