[AArch64] Insert subvector costs

An insert subvector under aarch64 can often be done as a single lane mov
operation. For example a v4i8 inserted into a v16i8 is a s-reg mov, so
long as the index is a multiple of 4. This teaches the cost model that,
using code copied over from the X86 backend.

Some of the costs (v16i16_4_0) are still high because they get matched
as a SK_Select, not an SK_InsertSubvector. D120879 has some codegen
tests for inserting subvectors, which I were added as
llvm/test/CodeGen/AArch64/insert-subvector.ll.

Differential Revision: https://reviews.llvm.org/D120880