[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

tools/opt/opt.cpp

@@ -22,6 +22,7 @@
 #include "llvm/Analysis/RegionPass.h"
 #include "llvm/Bitcode/BitcodeWriterPass.h"
 #include "llvm/CodeGen/CommandFlags.h"
+#include "llvm/InitializePasses.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/IRPrintingPasses.h"
 #include "llvm/IR/LLVMContext.h"
@@ -340,6 +341,9 @@ int main(int argc, char **argv) {
   initializeInstCombine(Registry);
   initializeInstrumentation(Registry);
   initializeTarget(Registry);
+  // For codegen passes, only passes that do IR to IR transformation are
+  // supported. For now, just add CodeGenPrepare.
+  initializeCodeGenPreparePass(Registry);
 
   cl::ParseCommandLineOptions(argc, argv,
     "llvm .bc -> .bc modular optimizer and analysis printer\n");