Add a workaround for the TailDuplicator compile time overhead

Author: dianqk

compiler/rustc_mir_transform/src/uninhabited_enum_branching.rs

@@ -118,9 +118,18 @@ impl<'tcx> MirPass<'tcx> for UninhabitedEnumBranching {
                     unreachable_targets.push(index);
                 }
             }
-
-            let replace_otherwise_to_unreachable = allowed_variants.len() <= 1
-                && !body.basic_blocks[targets.otherwise()].is_empty_unreachable();
+            let otherwise_is_empty_unreachable =
+                body.basic_blocks[targets.otherwise()].is_empty_unreachable();
+            // After resolving https://github.com/llvm/llvm-project/issues/78578,
+            // we can remove the limit on the number of successors.
+            let otherwise_is_last_variant = !otherwise_is_empty_unreachable
+                && allowed_variants.len() == 1
+                && !body.basic_blocks[targets.otherwise()]
+                    .terminator()
+                    .successors()
+                    .any(|bb| body.basic_blocks[bb].terminator().successors().count() >= 8);
+            let replace_otherwise_to_unreachable = otherwise_is_last_variant
+                || !otherwise_is_empty_unreachable && allowed_variants.is_empty();
 
             if unreachable_targets.is_empty() && !replace_otherwise_to_unreachable {
                 continue;
@@ -129,7 +138,6 @@ impl<'tcx> MirPass<'tcx> for UninhabitedEnumBranching {
             let unreachable_block = patch.unreachable_no_cleanup_block();
             let mut targets = targets.clone();
             if replace_otherwise_to_unreachable {
-                let otherwise_is_last_variant = !allowed_variants.is_empty();
                 if otherwise_is_last_variant {
                     #[allow(rustc::potential_query_instability)]
                     let last_variant = *allowed_variants.iter().next().unwrap();