[X86][FP16] Fix crash when lowering copysign for f16

[X86][FP16] Fix crash when lowering copysign for f16

This is to address the assertion fail reported in https://reviews.llvm.org/D107082#3635612
Not sure if it is a problem of promoting FCOPYSIGN + libcall FP_ROUND.
The promoting will set the rounding mode to 1 https://github.com/llvm/llvm-project/blob/a442c628882eb07fffff8c9f7c87a317af14555a/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp#L4810-L4814
While libcall cannot handle the rounding mode equals to 1 https://github.com/llvm/llvm-project/blob/a442c628882eb07fffff8c9f7c87a317af14555a/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp#L4324-L4328
So changing the action to Expand to workaround the problem.

Reviewed By: clementval, MaskRay

Differential Revision: https://reviews.llvm.org/D129294