[PowerPC] Lower IS_FPCLASS by test data class instruction

Power ISA 3.0 introduced new 'test data class' instructions, which
accept flags for: NaN/Infinity/Zero/Denormal. This instruction can be
used to implement custom lowering for llvm.is.fpclass, but some extra
bits provided by the intrinsic are missing (normal and QNaN/SNaN).

For those categories not natively supported, this patch uses a two-way
or three-way combination to implement correct behavior.

Reviewed By: sepavloff, shchenz

Differential Revision: https://reviews.llvm.org/D140381