[llvm] [SelectionDAG][Targets] Replace atomic_load_8/atomic_load_16 with atomic_load_*ext_8/atomic_load_*ext_16 where possible. (PR #137279)

[Sergei Barannikov via llvm-commits]

s-barannikov wrote:

Do I understand correctly that atomic loads were supposed to always be extended using `getExtendForAtomicOps()` if VT and MemVT are different?

https://github.com/llvm/llvm-project/pull/137279