[CodeGenPrepare] Move CodeGenPrepare into lib/CodeGen.

CodeGenPrepare uses extensively TargetLowering which is part of libLLVMCodeGen.
This is a layer violation which would introduce eventually a dependence on
CodeGen in ScalarOpts.

Move CodeGenPrepare into libLLVMCodeGen to avoid that.

Follow-up of <rdar://problem/15519855>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201912 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/Passes.h

@@ -370,6 +370,10 @@ namespace llvm {
   createMachineFunctionPrinterPass(raw_ostream &OS,
                                    const std::string &Banner ="");
 
+  /// createCodeGenPreparePass - Transform the code to expose more pattern
+  /// matching during instruction selection.
+  FunctionPass *createCodeGenPreparePass(const TargetMachine *TM = 0);
+
   /// MachineLoopInfo - This pass is a loop analysis pass.
   extern char &MachineLoopInfoID;