[llvm] [LV] Support partial reduce subs/fsubs without a mul operand (PR #199664)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

> We have the following in TTI, is that still accurate in all cases?

I think the TTI should be correct (similar to how it is for the add case). During ISEL these will be lowered the same as a the `mul` case (with one operand fixed to `splat(1)`).

https://github.com/llvm/llvm-project/pull/199664