[llvm] [SLP] Fix canConvertToFMA operand selection and fmul costing (PR #216425)

[Dmitry Sidorov via llvm-commits]

MrSidims wrote:

@alexey-bataev apologies, I missed [this comment](https://github.com/llvm/llvm-project/pull/210399#discussion_r3624591969) on the previous PR, but basically fix for canConvertToFMA was planned regardless. 

https://github.com/llvm/llvm-project/pull/216425