Revert "[maglev] Fix dead fallthrough merging"

This reverts commit c7aef552.

Reason for revert: Merged the wrong CL for the fix.

Original change's description:
> [maglev] Fix dead fallthrough merging
>
> Add a method which optionally merges dead fallthrough paths, in case the
> iteration in EmitUnconditionalDeopt reaches a merge point that is live
> from another jump but dead on the fallthrough.
>
> Bug: v8:7700
> Change-Id: Ie505cd5356fcf70208f2f6d3e52b805956485f74
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3663086
> Auto-Submit: Leszek Swirski <leszeks@chromium.org>
> Commit-Queue: Toon Verwaest <verwaest@chromium.org>
> Reviewed-by: Toon Verwaest <verwaest@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#80878}

Bug: v8:7700
Change-Id: I75a21777aecfa08138fcc25a882ae109f3409159
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3687649
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#80917}

src/maglev/maglev-graph-builder.h

@@ -151,10 +151,6 @@ class MaglevGraphBuilder {
         // Loops that are unreachable aside from their back-edge are going to
         // be entirely unreachable, thanks to irreducibility.
         if (!merge_states_[iterator_.current_offset()]->is_unreachable_loop()) {
-          // We might be falling through into this merge state, so merge in that
-          // fallthrough as dead, if necessary.
-          merge_states_[iterator_.current_offset()]->MaybeMergeDeadFallthrough(
-              *compilation_unit_, iterator_.current_offset());
           break;
         }
       }

src/maglev/maglev-interpreter-frame-state.h

@@ -325,18 +325,6 @@ class MergePointInterpreterFrameState {
         });
   }
 
-  // Merges a dead fallthrough framestate (e.g. one which has been early
-  // terminated with a deopt), if that merge is needed. It is needed when there
-  // is exactly one missing predecessor.
-  void MaybeMergeDeadFallthrough(const MaglevCompilationUnit& compilation_unit,
-                                 int merge_offset) {
-    // We've seen all predecessors, so we're good.
-    if (predecessors_so_far_ == predecessor_count_) return;
-    // Otherwise exactly one should be missing.
-    DCHECK_EQ(predecessors_so_far_, predecessor_count_ - 1);
-    MergeDead(compilation_unit, merge_offset);
-  }
-
   // Merges a dead loop framestate (e.g. one where the block containing the
   // JumpLoop has been early terminated with a deopt).
   void MergeDeadLoop() {
@@ -368,10 +356,7 @@ class MergePointInterpreterFrameState {
   }
 
   bool is_unreachable_loop() const {
-    // We should have either merged or removed all predecessors, except maybe
-    // the fallthrough.
-    DCHECK_GE(predecessors_so_far_, predecessor_count_ - 1);
-    DCHECK_LE(predecessors_so_far_, predecessor_count_);
+    DCHECK_EQ(predecessors_so_far_, predecessor_count_);
     // If there is only one predecessor, and it's not set, then this is a loop
     // merge with no forward control flow entering it.
     return predecessor_count_ == 1 &&