[llvm] [SDAG] Read-only intrinsics must have WillReturn attribute to be treated as loads (PR #99999)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I think the bpf.load intrinsics are in a similar state to the nvvm.ldu/ldg intrinsics, which might be why the bpf test change is needed. Those should probably have IntrWillReturn

Most intrinsics should have been migrated to DefaultAttrsIntrinsic, but that was never completed 

https://github.com/llvm/llvm-project/pull/99999