[Bug target/103784] suboptimal code for returning bool value on target ppc

["segher at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103784

--- Comment #12 from Segher Boessenkool <segher at gcc dot gnu.org> ---
What David says :-)

We really could use something good for this, it has been a problem for all
GCC targets since forever; it hurts rs6000 more than most though.

Before RA this is a diamond, one side does the 0/1, the other the always 0.
After the join it gets an AND with 1 (not an extend; the effect is similar
of course).  Shrink-wrapping gets rid of the join (duplicates the tail code
to both branches) but does not optimise the result of that, which gives us
the silly li 3,0;clrldi 3,3,63 (and the other side does not need the shift
either, but doesn't look quite as silly :-) ).

It is not unlikely this would work better if we had no QImode thing for the
bool; even SImode might work better already, but DImode would be best (in
the ABI everything is passed in full registers always, so something has to
set the upper bits somewhere).