[maglev] Fix SetPendingMessage

Add return register to the allocator. SetPendingMessage is a ValueNode
and returns the previous pending message.

This was not identified before, because we do not support
exception handlers yet.

Bug: v8:7700
Change-Id: Ia03ea609e85297198792816edda22d4f1c79708c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3849652
Commit-Queue: Victor Gomes <victorgomes@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Victor Gomes <victorgomes@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82654}

src/maglev/maglev-ir.cc

@@ -2421,8 +2421,9 @@ void LogicalNot::GenerateCode(MaglevCodeGenState* code_gen_state,
   }
 }
 
-void SetPendingMessage::AllocateVreg(MaglevVregAllocationState*) {
+void SetPendingMessage::AllocateVreg(MaglevVregAllocationState* vreg_state) {
   UseRegister(value());
+  DefineAsRegister(vreg_state, this);
 }
 
 void SetPendingMessage::GenerateCode(MaglevCodeGenState* code_gen_state,