[ignition] Eliminate hole checks where statically possible for loads and stores

src/ast/variables.h

Index: src/ast/variables.h
diff --git a/src/ast/variables.h b/src/ast/variables.h
index 47ff313286ef044536299a013a7b75858379aecf..44cfd412cfbac0d5c9fc0e7b95b44ac0ce20ce0b 100644
--- a/src/ast/variables.h
+++ b/src/ast/variables.h
@@ -100,14 +100,19 @@ class Variable final : public ZoneObject {
 
   int index() const { return index_; }
 
+  // Sentinel index values for module exports and imports.
+  enum { kModuleExportIndex, kModuleImportIndex };
+
   bool IsExport() const {
     DCHECK(location() == VariableLocation::MODULE);
-    return index() == 0;
+    return index() == kModuleExportIndex;
   }
 
   void AllocateTo(VariableLocation location, int index) {
     DCHECK(IsUnallocated() ||
            (this->location() == location && this->index() == index));
+    DCHECK_IMPLIES(location == VariableLocation::MODULE,
+                   index == kModuleExportIndex || index == kModuleImportIndex);
     bit_field_ = LocationField::update(bit_field_, location);
     DCHECK_EQ(location, this->location());
     index_ = index;