[llvm] [AArch64] Implement latency costs for loads (PR #201567)

[John Brawn via llvm-commits]

john-brawn-arm wrote:

This is being done for #197959 - the plan is to to enable it on AArch64 in-order CPUs, but that needs accurate load latency. I also need to do getInterleavedMemoryOpCost, which I'll do in a separate PR.

https://github.com/llvm/llvm-project/pull/201567