Fix bad code-generation for nested-ifs in for loops.

lib/compiler/implementation/ssa/codegen_helpers.dart

Index: lib/compiler/implementation/ssa/codegen_helpers.dart
diff --git a/lib/compiler/implementation/ssa/codegen_helpers.dart b/lib/compiler/implementation/ssa/codegen_helpers.dart
index 06caaa0ba493a2f79480acd4dbcb35b633010bfe..19e177c5b9038cc76f12f28b2181950f049b6d3b 100644
--- a/lib/compiler/implementation/ssa/codegen_helpers.dart
+++ b/lib/compiler/implementation/ssa/codegen_helpers.dart
@@ -305,6 +305,7 @@ class SsaConditionMerger extends HGraphVisitor {
       if (otherIf.joinBlock !== end) {
         // This could be a join block that just feeds into our join block.
         HBasicBlock otherJoin = otherIf.joinBlock;
+        if (otherJoin.first != otherJoin.last) return;
         if (otherJoin.successors.length != 1) return;
         if (otherJoin.successors[0] != end) return;
         if (otherJoin.phis.isEmpty()) return;