[AArch64][TTI] Allow mixed-extension partial reductions with +dotprod  (#199762)

With the backend now lowering SUMLA via two udot products on targets
that have +dotprod (#199761), lower the cost on targets without +i8mm.

PR: https://github.com/llvm/llvm-project/pull/199762
GitOrigin-RevId: 68ce9d311c8557a22005f676b1fb93a1bf87aefd
2 files changed