rs6000: Backports of mode promote patches to GCC 11

[Segher Boessenkool]

Hi!

I have backported 9080a3bf2329, a3f6bd789149, and f0529d96f567 to the
GCC 11 branch.  This solves PR102062, but is a >2% performance win for
such a trivial patch, too, which is enough reason on its own :-)


Segher