[X86][LegalizeTypes] Add SoftPromoteHalf support STRICT_FP_EXTEND and STRICT_FP_ROUND

This adds a strict version of FP16_TO_FP and FP_TO_FP16 and uses
them to implement soft promotion for the half type. This is
enough to provide basic support for __fp16 with strictfp.

Add the necessary X86 support to use VCVTPS2PH/VCVTPH2PS when F16C
is enabled.