[llvm] [NVPTX] Load/Store/Fence syncscope support (PR #106101)

[via llvm-commits]

gonzalobg wrote:

Have added the two `llvm_unreachable`.

For different sub-sequent PRs:
- There are many TODOs in the tests, that will get incrementally fixed over time (with smaller scoped changes).
- I should improve the Printer APIs to use `StringRef`.
- I should add a `getSyncScope(ID)` API to context, to get a single `StringRef`.
- I should explore merging the two `AddressSpace` enums into one. 

https://github.com/llvm/llvm-project/pull/106101