R600: Implement isNarrowingProfitable.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204658 91177308-0d34-0410-b5e6-96231b3b80d8
In Release modes, Visual Studio complains that the Operator destructor in User.cpp
never returns, which is true by design. 

Initially assumed that the reason is llvm_unreachable being dependent on NDEBUG.

However, even if llvm_unreachable is replaced by __assume(false), VC still warns in
Release modes but not in Debug modes...

The real reason turned out to be optimization flags.
With /Od in Debug modes the warning is not issued whereas with /O1 it is.

I could not find any documentation to this effect, but it is reproducable:

Try compiling http://msdn.microsoft.com/en-us/library/khwfyc5d(v=vs.90).aspx
with /O1 and then with /Od.




git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204659 91177308-0d34-0410-b5e6-96231b3b80d8