[NVPTX] Add NVVMReflect pass to allow compile-time selection of

[NVPTX] Add NVVMReflect pass to allow compile-time selection of
specific code paths.

This allows us to write code like:

  if (__nvvm_reflect("FOO"))
    // Do something
  else
    // Do something else

and compile into a library, then give "FOO" a value at kernel
compile-time so the check becomes a no-op.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178416 91177308-0d34-0410-b5e6-96231b3b80d8