[PowerPC] Handle f16 as a storage type only

The PPC back end currently crashes (fails to select) with f16 input. This patch
expands it on subtargets prior to ISA 3.0 (Power9) and uses the HW conversions
on Power9.

Fixes https://bugs.llvm.org/show_bug.cgi?id=39865

Differential revision: https://reviews.llvm.org/D68237