Enhance address mode recovery

src/IceFixups.h

Index: src/IceFixups.h
diff --git a/src/IceFixups.h b/src/IceFixups.h
index 303e592bb3d35e582eadb5141139f481e10f8a8d..36c333719c4665e7bc09633392ee526609873d09 100644
--- a/src/IceFixups.h
+++ b/src/IceFixups.h
@@ -31,8 +31,14 @@ struct AssemblerFixup {
 public:
   AssemblerFixup() = default;
   AssemblerFixup(const AssemblerFixup &) = default;
-  intptr_t position() const { return position_; }
-  void set_position(intptr_t Position) { position_ = Position; }
+  intptr_t position() const {
+    assert(position_was_set_);
+    return position_;
+  }
+  void set_position(intptr_t Position) {
+    position_ = Position;
+    position_was_set_ = true;
+  }
 
   FixupKind kind() const { return kind_; }
   void set_kind(FixupKind Kind) { kind_ = Kind; }
@@ -50,6 +56,7 @@ public:
                       bool IsPCRel) const;
 
 private:
+  bool position_was_set_ = false;
   intptr_t position_ = 0;
   FixupKind kind_ = 0;
   const Constant *value_ = nullptr;