[llvm] [AArch64] Add costs for ST3 and ST4 instructions, modelled as store(shuffle). (PR #87934)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

LGTM - my only remaining thought is whether we need the Args argument now we have CxtI?

https://github.com/llvm/llvm-project/pull/87934