[AArch64] Generalize costing for FP16 instructions (#150033)
This extracts the code for modelling a fp16 operation as
`fptrunc(fpop(fpext,fpext))` into a new function named
getFP16BF16PromoteCost so that it can be reused by the
arithmetic instructions. The function takes a lambda to
calculate the cost of the operation with the promoted type.