[vm/compiler] Allow Slot::Equals to be used with native slots.

Before this change, Slot::Equals would hit the default UNREACHABLE()
case if used to compare two equal native slots. That's surprising and
also unavoidable if code comparing arbitrary slots wants to check Slot
equality, since there's no existing predicate to determine whether an
arbitrary slot is native.

TEST=ci

Cq-Include-Trybots: luci.dart.try:vm-aot-linux-debug-x64-try,vm-aot-mac-release-arm64-try,vm-aot-mac-product-arm64-try,vm-aot-linux-product-x64-try,vm-aot-dwarf-linux-product-x64-try,vm-linux-debug-x64-try,vm-mac-debug-arm64-try,vm-mac-release-arm64-try
Change-Id: Ia794a33fc607013bf35435b0427ceb02bdae4b0d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/419560
Reviewed-by: Daco Harkes <[email protected]>
Commit-Queue: Tess Strickland <[email protected]>

Author: sstrickl

runtime/vm/compiler/backend/slot.cc

@@ -469,6 +469,9 @@ bool Slot::Equals(const Slot& other) const {
   }
 
   switch (kind_) {
+#define NATIVE_SLOT_CASE(CN, __, FN, ___, ____) case Kind::k##CN##_##FN:
+    NATIVE_SLOTS_LIST(NATIVE_SLOT_CASE)
+#undef NATIVE_SLOT_CASE
     case Kind::kTypeArguments:
     case Kind::kTypeArgumentsIndex:
     case Kind::kArrayElement: