Add a workaround for the TailDuplicator compile time overhead

Author: dianqk

compiler/rustc_mir_transform/src/uninhabited_enum_branching.rs

@@ -130,7 +130,13 @@ impl<'tcx> MirPass<'tcx> for UninhabitedEnumBranching {
             let mut targets = targets.clone();
             if replace_otherwise_to_unreachable {
                 let otherwise_is_last_variant = !allowed_variants.is_empty();
-                if otherwise_is_last_variant {
+                // After resolving https://github.com/llvm/llvm-project/issues/78578, we can remove the limit on the number of successors.
+                if otherwise_is_last_variant
+                    && !body.basic_blocks[targets.otherwise()]
+                        .terminator()
+                        .successors()
+                        .any(|bb| body.basic_blocks[bb].terminator().successors().count() >= 8)
+                {
                     #[allow(rustc::potential_query_instability)]
                     let last_variant = *allowed_variants.iter().next().unwrap();
                     targets.add_target(last_variant, targets.otherwise());