[NFC] Header cleanup

Removed some unused headers, replaced some headers with forward class declarations.

Found using simple scripts like this one:
clear && ack --cpp -l '#include "llvm/ADT/IndexedMap.h"' | xargs grep -L 'IndexedMap[<]' | xargs grep -n --color=auto 'IndexedMap'

Patch by Eugene Kosov <claprix@yandex.ru>

Differential Revision: http://reviews.llvm.org/D19219

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 266595
diff --git a/llvm/lib/Target/Hexagon/HexagonGenInsert.cpp b/llvm/lib/Target/Hexagon/HexagonGenInsert.cpp
index 5fa7725..f04ac38 100644
--- a/llvm/lib/Target/Hexagon/HexagonGenInsert.cpp
+++ b/llvm/lib/Target/Hexagon/HexagonGenInsert.cpp
@@ -9,11 +9,8 @@
 
 #define DEBUG_TYPE "hexinsert"
 
-#include "llvm/Pass.h"
-#include "llvm/PassRegistry.h"
 #include "llvm/ADT/BitVector.h"
 #include "llvm/ADT/DenseMap.h"
-#include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/PostOrderIterator.h"
 #include "llvm/CodeGen/MachineDominators.h"
 #include "llvm/CodeGen/MachineFunction.h"
@@ -21,10 +18,12 @@
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/IR/Constants.h"
+#include "llvm/Pass.h"
+#include "llvm/PassRegistry.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"
-#include "llvm/Support/raw_ostream.h"
 #include "llvm/Support/Timer.h"
+#include "llvm/Support/raw_ostream.h"
 #include "llvm/Target/TargetMachine.h"
 #include "llvm/Target/TargetRegisterInfo.h"
 
@@ -33,7 +32,6 @@
 #include "HexagonTargetMachine.h"
 #include "HexagonBitTracker.h"
 
-#include <map>
 #include <vector>
 
 using namespace llvm;