MIPS: Fix bugs in IfBuilder and improve functionality

src/mips/lithium-mips.cc

Index: src/mips/lithium-mips.cc
diff --git a/src/mips/lithium-mips.cc b/src/mips/lithium-mips.cc
index afa806c4ec7d8e537f9114ea296055bb1b11c64d..0d12c458b8ae6e36d7be7441eb49a7371e5d0343 100644
--- a/src/mips/lithium-mips.cc
+++ b/src/mips/lithium-mips.cc
@@ -823,11 +823,15 @@ void LChunkBuilder::DoBasicBlock(HBasicBlock* block, HBasicBlock* next_block) {
     HEnvironment* last_environment = pred->last_environment();
     for (int i = 0; i < block->phis()->length(); ++i) {
       HPhi* phi = block->phis()->at(i);
-      last_environment->SetValueAt(phi->merged_index(), phi);
+      if (phi->merged_index() < last_environment->length()) {
+        last_environment->SetValueAt(phi->merged_index(), phi);
+      }
     }
     for (int i = 0; i < block->deleted_phis()->length(); ++i) {
-      last_environment->SetValueAt(block->deleted_phis()->at(i),
-                                   graph_->GetConstantUndefined());
+      if (block->deleted_phis()->at(i) < last_environment->length()) {
+        last_environment->SetValueAt(block->deleted_phis()->at(i),
+                                     graph_->GetConstantUndefined());
+      }
     }
     block->UpdateEnvironment(last_environment);
     // Pick up the outgoing argument count of one of the predecessors.