[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.

llvm-svn: 178416