[maglev] Fix double register merging

DoubleRegister RegisterMerge could get assigned a Tagged representation
operand if the node in the merge was null.

Also, we had a wrong DCHECK when materialising these moves, as a result
of templatifying the gap move implementation.

Bug: v8:7700
Change-Id: I2a425a6bedf4c67e8acf6c30c877e055b6445b12
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3783994
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82018}

src/maglev/maglev-code-generator.cc

@@ -273,7 +273,6 @@ class MaglevCodeGeneratingNodeProcessor {
                      RegisterT target_reg,
                      RegisterMovesT<RegisterT>& register_moves,
                      RegisterReloadsT<RegisterT>& register_reloads) {
-    DCHECK(!source.IsDoubleRegister());
     if (source.IsAnyRegister()) {
       // For reg->reg moves, don't emit the move yet, but instead record the
       // move in the set of parallel register moves, to be resolved later.

src/maglev/maglev-regalloc.cc

@@ -1493,9 +1493,12 @@ void StraightForwardRegisterAllocator::MergeRegisterValues(ControlNode* control,
     RegisterMerge* merge;
     LoadMergeState(state, &node, &merge);
 
-    MachineRepresentation mach_repr = node == nullptr
+    // This isn't quite the right machine representation for Int32 nodes, but
+    // those are stored in the same registers as Tagged nodes so in this case it
+    // doesn't matter.
+    MachineRepresentation mach_repr = std::is_same_v<decltype(reg), Register>
                                           ? MachineRepresentation::kTagged
-                                          : node->GetMachineRepresentation();
+                                          : MachineRepresentation::kFloat64;
     compiler::AllocatedOperand register_info = {
         compiler::LocationOperand::REGISTER, mach_repr, reg.code()};