[maglev] Fix regalloc DCHECK

The comment was right but the actual condition wasn't. We should check
whether the value is _not_ loadable.

Bug: v8:7700
Change-Id: I1c721a56da5860c73c8179406abb1d3a8b9d08f6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3755111
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81638}

src/maglev/maglev-regalloc.cc

@@ -1510,7 +1510,7 @@ void StraightForwardRegisterAllocator::MergeRegisterValues(ControlNode* control,
 
       // If there's a value in the incoming state, that value is either
       // already spilled or in another place in the merge state.
-      if (incoming != nullptr && incoming->is_loadable()) {
+      if (incoming != nullptr && !incoming->is_loadable()) {
         DCHECK(IsInRegister(target_state, incoming));
       }
       return;