Handle LLVM intrinsic llvm.is_fpclass

[alan-baker]

LLVM has started inferring llvm.is_fpclass (see https://llvm.org/docs/LangRef.html#llvm-is-fpclass) in some cases including the following CTS tests:

• half/vstore_half_rtp
• half/vstorea_half_rtp

In those cases it only used 0x40 (positive zero). Those tests passed with either zero though (e.g. using fcmp oeq 0). The remaining functionality should be implemented and tested.