[AArch64] More accurately model cost of partial reductions (#181707) With #181706 using the cost-model to decide whether using partial reductions is profitable, we need to more accurately represent the cost of certain partial reduction operations: * Reflect the fact that *MLALB/T instructions can be used for 16-bit -> 32-bit partial reductions (or *MLAL/MLAL2 for NEON). * Calculate the cost of expanding the partial reduction in ISel for reductions that don't have an explicit instruction, rather than returning a random number. For sub-reductions we scale the cost to make them slightly cheaper, so that they're still candidates for forming cdot operations.
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