[vm/compiler] Do not assume that LoadField is performed from a record of a compatible shape

Compiler can see an unreachable code like this:

v1 <- Constant(Record (C, C))
if v1 is (C, C, C) {
  v2 <- LoadField (v1.$3)
}

So it should check that record type of the receiver has enough fields
before querying field type in LoadField.

TEST=co19/LanguageFeatures/Patterns/matching_record_*

Fixes #51767
Issue #49755

Cq-Include-Trybots: luci.dart.try:vm-kernel-precomp-nnbd-linux-release-x64-try
Change-Id: Iac42c8edf3d22dce0061b6d75e8bdf51b7bf88e6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/289540
Auto-Submit: Alexander Markov <[email protected]>
Reviewed-by: Martin Kustermann <[email protected]>
Commit-Queue: Martin Kustermann <[email protected]>

Author: alexmarkov

runtime/vm/compiler/backend/type_propagator.cc

@@ -1688,10 +1688,14 @@ CompileType LoadFieldInstr::ComputeType() const {
     }
     const AbstractType* instance_type = instance()->Type()->ToAbstractType();
     if (instance_type->IsRecordType()) {
-      const auto& field_type = AbstractType::ZoneHandle(
-          RecordType::Cast(*instance_type).FieldTypeAt(index));
-      return CompileType::FromAbstractType(field_type, CompileType::kCanBeNull,
-                                           CompileType::kCannotBeSentinel);
+      const auto& record_type = RecordType::Cast(*instance_type);
+      if (index < record_type.NumFields()) {
+        const auto& field_type =
+            AbstractType::ZoneHandle(record_type.FieldTypeAt(index));
+        return CompileType::FromAbstractType(field_type,
+                                             CompileType::kCanBeNull,
+                                             CompileType::kCannotBeSentinel);
+      }
     }
   }
   CompileType type = slot().ComputeCompileType();