Revert "[NFC-I] Remove hack for fp-classification builtins"

This reverts commit b1e542f3.

The original 'hack' didn't chop out fp-16 to double conversions, so
systems that use FP16ConversionIntrinsics end up in IR-CodeGen with an
i16 type isntead of a float type (like PPC64-BE).  The bots noticed
this.

Reverting until I figure out how to fix this