[dart2js] new-rti: Fix generator header-body merge optimization

Change-Id: Id4f038415990f5379c1a01637b53c398f5325449
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/112270
Reviewed-by: Mayank Patke <fishythefish@google.com>
Commit-Queue: Stephen Adams <sra@google.com>

pkg/compiler/lib/src/ssa/builder_kernel.dart

@@ -1426,6 +1426,7 @@ class KernelSsaGraphBuilder extends ir.Visitor {
     if (block.id != 1) return false;
     for (HInstruction node = block.first; node != null; node = node.next) {
       if (node is HGoto) continue;
+      if (node is HLoadType) continue; // Orphaned if check is redundant.
       return false;
     }
     return true;

pkg/compiler/lib/src/ssa/type_builder.dart

@@ -85,6 +85,9 @@ abstract class TypeBuilder {
     if (other is HTypeConversion && other.isRedundant(builder.closedWorld)) {
       return original;
     }
+    if (other is HAsCheck && other.isRedundant(builder.closedWorld)) {
+      return original;
+    }
     return other;
   }