Fix generateAtUseSite for complex HIs checks

pkg/compiler/lib/src/ssa/codegen_helpers.dart

Index: pkg/compiler/lib/src/ssa/codegen_helpers.dart
diff --git a/pkg/compiler/lib/src/ssa/codegen_helpers.dart b/pkg/compiler/lib/src/ssa/codegen_helpers.dart
index c3e2bb6a3dd9840d421e1418265a07bc5dfb598d..34d156b292898108a4e11f060c93f6c951c8e543 100644
--- a/pkg/compiler/lib/src/ssa/codegen_helpers.dart
+++ b/pkg/compiler/lib/src/ssa/codegen_helpers.dart
@@ -440,11 +440,18 @@ class SsaInstructionMerger extends HBaseVisitor {
 
   void visitIs(HIs instruction) {
     // In the general case the input might be used multple multiple times, so it
-    // must not be set generate at use site.  If the code will generate
-    // 'instanceof' then we can generate at use site.
+    // must not be set generate at use site.
+
+    // If the code will generate 'instanceof' then we can generate at use site.
     if (instruction.useInstanceOf) {
       analyzeInputs(instruction, 0);
     }
+
+    // Compound and variable checks use a separate instruction to compute the
+    // result.
+    if (instruction.isCompoundCheck || instruction.isVariableCheck) {
+      analyzeInputs(instruction, 0);
+    }
   }
 
   // A bounds check method must not have its first input generated at use site,