[PowerPC] Add missing handling for half precision

The fix for PR39865 took care of some of the handling for half precision
but it missed a number of issues that still exist. This patch fixes the
remaining issues that cause crashes in the PPC back end.

Fixes: https://bugs.llvm.org/show_bug.cgi?id=45776

Differential revision: https://reviews.llvm.org/D79283

(cherry picked from commit 1a493b0f)