[PowerPC] Do not emit HW loop with half precision operations

If a loop has any operations on half precision values, there will be calls to
library functions on Power8. Even on Power9, there is a small subset of
instructions that are actually supported for the type.

This patch disables HW loops whenever any operations on the type are found
(other than the handfull of supported ones when compiling for Power9). Fixes a
few PR's opened by Julia:

https://bugs.llvm.org/show_bug.cgi?id=48785
https://bugs.llvm.org/show_bug.cgi?id=48786
https://bugs.llvm.org/show_bug.cgi?id=48519

Differential revision: https://reviews.llvm.org/D94980