decoder/a64: Don't rearrange unrelated decoders

src/frontend/A64/decoder/a64.h

@@ -30,8 +30,11 @@ std::vector<Matcher<Visitor>> GetDecodeTable() {
 #undef INST
     };
 
-    // If a matcher has more bits in its mask it is more specific, so it should come first.
     std::stable_sort(table.begin(), table.end(), [](const auto& matcher1, const auto& matcher2) {
+        // If the matchers aren't related, keep their relative positions the same.
+        if ((matcher1.GetMask() & ~matcher2.GetMask()) && (~matcher1.GetMask() & matcher2.GetMask()))
+            return false;
+        // If a matcher has more bits in its mask it is more specific, so it should come first.
         return Common::BitCount(matcher1.GetMask()) > Common::BitCount(matcher2.GetMask());
     });
 

src/frontend/A64/translate/impl/impl.cpp

@@ -22,8 +22,8 @@ bool TranslatorVisitor::UnpredictableInstruction() {
 }
 
 bool TranslatorVisitor::DecodeError() {
-    // TODO: This is an internal error.
-    return UnallocatedEncoding();
+    UNREACHABLE();
+    return false;
 }
 
 bool TranslatorVisitor::ReservedValue() {