[RS6000] Fix <bd>_<mode> and <bd>tf_<mode> splitters

[Alan Modra]

On Sat, Feb 16, 2019 at 11:14:11AM -0600, Segher Boessenkool wrote:
> Okay, thanks!  For trunk as well as backports.

Applied to trunk.  gcc-8 is frozen until after the 8.3 release so I'll
have to wait a while for backports.  There is some chance we'll hit
ICEs on 8.3 like we occasionally did when reload used to put a loop
counter in an fpr..

-- 
Alan Modra
Australia Development Lab, IBM