[ARM] Enable ARMCodeGenPrepare by default

[samparker]

Summary

We've had the pass enabled downstream for a couple of weeks and it seems to be okay, so enable it by default.

[SjoerdMeijer]

We've had the pass enabled downstream for a couple of weeks and it seems to be okay, so enable it by default.

Agreed, and I think you already had the green light to flip the switch again in:

http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20180820/581656.html

lib/Target/ARM/ARMCodeGenPrepare.cpp

 #include "llvm/Support/Casting.h"
 #include "llvm/Support/CommandLine.h"
 
 #define DEBUG_TYPE "arm-codegenprepare"
 
 using namespace llvm;
 
 static cl::opt<bool>
-DisableCGP("arm-disable-cgp", cl::Hidden, cl::init(true),
+DisableCGP("arm-disable-cgp", cl::Hidden, cl::init(false),
            cl::desc("Disable ARM specific CodeGenPrepare pass"));
 
 static cl::opt<bool>
 EnableDSP("arm-enable-scalar-dsp", cl::Hidden, cl::init(false),
           cl::desc("Use DSP instructions for scalar operations"));
 
 static cl::opt<bool>
 EnableDSPWithImms("arm-enable-scalar-dsp-imms", cl::Hidden, cl::init(false),