[llvm] [Targets] Migrate from atomic_load_8/16/32/64 to atomic_load_nonext_8/16/32/64. NFC (PR #137428)

[Craig Topper via llvm-commits]

topperc wrote:

> This is more question to #137401 which I slept through. Why do we need to spell out `_nonext`? It seems to be implied by _non-atomic_ loads. Are atomic loads somewhat different in this regard?

If I add the IsNonExtLoad flag to atomic_load_8/16/32/64 it will cause "cannot select" failures for out of tree target that haven't implemented #137279. Spelling it out explicitly and deleting the old names give them build time failures instead. I figured the build time failure was preferable to a test failure.

https://github.com/llvm/llvm-project/pull/137428