[X86] Refactor X86ScalarSSEf16/32/64 with hasFP16/SSE1/SSE2. NFCI

This is used for f16 emulation. We emulate f16 for SSE2 targets and
above. Refactoring makes the future code to be more clean.

Reviewed By: LuoYuanke

Differential Revision: https://reviews.llvm.org/D122475