[AArch64][SVE]Add cost model for masked gather and scatter for scalable vector.

    A new TTI interface has been added 'Optional <unsigned>getMaxVScale' that
    returns the maximum vscale for a given target.
    When known getMaxVScale is used to compute the cost of masked gather scatter
    for scalable vector.

    Depends on D92094

    Differential Revision: https://reviews.llvm.org/D93030