Handle fallthrough, kill and nop in type determination.

These statements have no expressions to determine so they just return
true.

Bug: tint:5
Change-Id: I639bce9c254c12357ac3a1d2a9cfaaee437716c0
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/18833
Reviewed-by: David Neto <dneto@google.com>

src/type_determiner.cc

@@ -96,6 +96,15 @@ bool TypeDeterminer::DetermineResultType(ast::Statement* stmt) {
     return DetermineResultType(e->condition()) &&
            DetermineResultType(e->body());
   }
+  if (stmt->IsFallthrough()) {
+    return true;
+  }
+  if (stmt->IsKill()) {
+    return true;
+  }
+  if (stmt->IsNop()) {
+    return true;
+  }
 
   error_ = "unknown statement type for type determination";
   return false;