[llvm] [LV] Handle FSub Partial Reductions (PR #191186)

[Sander de Smalen via llvm-commits]

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sdesmalen-arm wrote:

At the moment this can't be triggered because none of the targets return a valid cost for `FSub`. Maybe an assert that the opcode isn't an FSub suffices for this PR? (with a negative test for the fadd+fsub case that wouldn't yet result in a partial reduction).

https://github.com/llvm/llvm-project/pull/191186