shader/memory: Implement ATOM.ADD

ATOM operates atomically on global memory. For now only add ATOM.ADD
since that's what was found in commercial games.

This asserts for ATOM.ADD.S32 (handling the others as unimplemented),
although ATOM.ADD.U32 shouldn't be any different.

This change forces us to change the default type on SPIR-V storage
buffers from float to uint. We could also alias the buffers, but it's
simpler for now to just use uint. While we are at it, abstract the code
to avoid repetition.

src/video_core/shader/node.h

@@ -162,7 +162,7 @@ enum class OperationCode {
     AtomicImageXor,      /// (MetaImage, int[N] coords) -> void
     AtomicImageExchange, /// (MetaImage, int[N] coords) -> void
 
-    UAtomicAdd, /// (smem, uint) -> uint
+    AtomicAdd, /// (memory, {u}int) -> {u}int
 
     Branch,         /// (uint branch_target) -> void
     BranchIndirect, /// (uint branch_target) -> void