Add flag --trace-phase to selectively produce IR trace output.

src/arm/lithium-arm.cc

Index: src/arm/lithium-arm.cc
===================================================================
--- src/arm/lithium-arm.cc	(revision 10869)
+++ src/arm/lithium-arm.cc	(working copy)
@@ -440,7 +440,7 @@
 
 
 void LChunk::MarkEmptyBlocks() {
-  HPhase phase("Mark empty blocks", this);
+  HPhase phase("L Mark empty blocks", this);
   for (int i = 0; i < graph()->blocks()->length(); ++i) {
     HBasicBlock* block = graph()->blocks()->at(i);
     int first = block->first_instruction_index();
@@ -552,7 +552,7 @@
 LChunk* LChunkBuilder::Build() {
   ASSERT(is_unused());
   chunk_ = new(zone()) LChunk(info(), graph());
-  HPhase phase("Building chunk", chunk_);
+  HPhase phase("L Building chunk", chunk_);
   status_ = BUILDING;
   const ZoneList<HBasicBlock*>* blocks = graph()->blocks();
   for (int i = 0; i < blocks->length(); i++) {