reader/wgsl: Generate ForLoopStatements

Instead of LoopStatements.

Update the writers to handle these.

Fixed: tint:952
Change-Id: Ibef66e133224810efc28c224d910b5e21f71f8d6
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/57203
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: James Price <jrprice@google.com>

src/transform/spirv.cc

@@ -29,6 +29,7 @@
 #include "src/sem/variable.h"
 #include "src/transform/external_texture_transform.h"
 #include "src/transform/fold_constants.h"
+#include "src/transform/for_loop_to_loop.h"
 #include "src/transform/inline_pointer_lets.h"
 #include "src/transform/manager.h"
 #include "src/transform/simplify.h"
@@ -50,6 +51,7 @@ Output Spirv::Run(const Program* in, const DataMap& data) {
   manager.Add<Simplify>();           // Required for arrayLength()
   manager.Add<FoldConstants>();
   manager.Add<ExternalTextureTransform>();
+  manager.Add<ForLoopToLoop>();  // Must come after ZeroInitWorkgroupMemory
   auto transformedInput = manager.Run(in, data);
 
   auto* cfg = data.Get<Config>();