dart2js: Move always-taken branch code to dominate join

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

Index: pkg/compiler/lib/src/ssa/optimize.dart
diff --git a/pkg/compiler/lib/src/ssa/optimize.dart b/pkg/compiler/lib/src/ssa/optimize.dart
index 4b5679b33c1a4fe9666c5feaa8d8b5992a7500c2..b15ef7b0c1cc32754d03aa7f9b53ecb988affe54 100644
--- a/pkg/compiler/lib/src/ssa/optimize.dart
+++ b/pkg/compiler/lib/src/ssa/optimize.dart
@@ -1623,6 +1623,7 @@ class SsaDeadCodeEliminator extends HGraphVisitor implements OptimizationPhase {
       instruction = previous;
     }
     block.forEachPhi(simplifyPhi);
+    evacuateTakenBranch(block);
   }
 
   void simplifyPhi(HPhi phi) {
@@ -1702,6 +1703,30 @@ class SsaDeadCodeEliminator extends HGraphVisitor implements OptimizationPhase {
     return null;
   }
 
+  /// If [block] is an always-taken branch, move the code from the taken branch
+  /// into [block]. This has the effect of making the instructions available for
+  /// further optimizations by moving them to a position that dominates the join
+  /// point of the if-then-else.
+  // TODO(29475): Delete dead blocks instead.
+  void evacuateTakenBranch(HBasicBlock block) {
+    if (!block.isLive) return;
+    HControlFlow branch = block.last;
+    if (branch is HIf) {
+      if (branch.thenBlock.isLive == branch.elseBlock.isLive) return;
+      assert(branch.condition.isConstant());
+      HBasicBlock target =
+          branch.thenBlock.isLive ? branch.thenBlock : branch.elseBlock;
+      HInstruction instruction = target.first;
+      while (!instruction.isControlFlow()) {
+        HInstruction next = instruction.next;
+        if (instruction is HTypeKnown && instruction.isPinned) break;
+        instruction.block.detach(instruction);
+        block.moveAtExit(instruction);
+        instruction = next;
+      }
+    }
+  }
+
   void cleanPhis() {
     L:
     for (HBasicBlock block in _graph.blocks) {