[D-runtime] core.bitop.bsr

[Jonathan M Davis]

Is there any reason that Phobos can't assume that core.bitop.bsr is defined? 
It's not versioned at all, so as far as I can tell, it will always exist. But 
for some reason std.utf.strideImpl currently checks for whether core.bitop.bsr 
is defined. I'd prefer to remove dead code like that. Does anyone know why it 
_wouldn't_ be dead code (i.e. why core.bitop.bsr wouldn't be defined)?

- Jonathan M Davis