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

This tries to add some costs for the shuffle in a ST3/ST4 instruction,
which are represented in LLVM IR as store(interleaving shuffle). In
order to detect the store, it needs to add a CxtI context instruction to
check the users of the shuffle. LD3 and LD4 are added, LD2 should be a
zip1 shuffle, which will be added in another patch.

It should help fix some of the regressions from #87510.