[TTI] Return a more sensible cost for histogram intrinsic. (#97397)

This is just an initial cost, making it invalid for any target which
doesn't specifically return a cost for now. Also adds an AArch64
specific cost check.

We will need to improve that later, e.g. by returning a scalarization
cost for generic targets and possibly introducing a new TTI method, at
least once LoopVectorize has changed it's cost model. The reason is
that the histogram intrinsic also effectively contains a gather and
scatter, and we will need details of the addressing to determine an
appropriate cost for that.

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