PowerPC supports `ppc_fp128`, which is not an IEEE floating point
type. The generic lowering of llvm.is_fpclass could not handle it
properly. This change extends the generic lowering code to
support `ppc_fp128`.
The change was tested on emulator using runtime tests from
https://reviews.llvm.org/D112933 and the patch for clang
https://reviews.llvm.org/D112932.
Differential Revision: https://reviews.llvm.org/D113908
