[llvm] Convert LoadExtActions to a map (PR #157627)

[Sam Parker via llvm-commits]

sparker-arm wrote:

> all the (indirect) uses of getLoadExtAction (including uses isLoadExtLegal[OrCustom] of) need to be made address space aware. 

I think this is all done now. I've fixed up the AMDGPU tests, as I figured that would be the best backend to begin with. I haven't figured out why the tests change though, but there aren't many. I do get a hard crash with a AArch64 SVE test though!

> any backends using extloads from anything other than address space 0 need fixing

Yeah, the main problem with this, current, implementation is that once you set an action for an explicit (non-zero) address space, I think this means that actions need to be set for all types :/

https://github.com/llvm/llvm-project/pull/157627