[vm/compiler] Restore old definition of BoxLanesInstr::ComputeType().

TEST=lib/typed_data/float32x4 lib/typed_data/int32x4 on RISCV.

Fixes: #55040
Cq-Include-Trybots: luci.dart.try:vm-aot-linux-debug-simriscv64-try,vm-linux-debug-simriscv64-try
Change-Id: I445195d12bbb209209d6958dbfcbfc634422ae6a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/354842
Commit-Queue: Tess Strickland <[email protected]>
Auto-Submit: Tess Strickland <[email protected]>
Commit-Queue: Martin Kustermann <[email protected]>
Reviewed-by: Martin Kustermann <[email protected]>

Author: sstrickl

runtime/vm/compiler/backend/type_propagator.cc

@@ -1807,7 +1807,17 @@ CompileType BoxInstr::ComputeType() const {
 }
 
 CompileType BoxLanesInstr::ComputeType() const {
-  return CompileType::FromUnboxedRepresentation(from_representation());
+  switch (from_representation()) {
+    case kUnboxedFloat:
+      return CompileType::FromCid(kFloat32x4Cid);
+    case kUnboxedDouble:
+      return CompileType::FromCid(kFloat64x2Cid);
+    case kUnboxedInt32:
+      return CompileType::FromCid(kInt32x4Cid);
+    default:
+      UNREACHABLE();
+      return CompileType::Dynamic();
+  }
 }
 
 CompileType ExtractNthOutputInstr::ComputeType() const {