CodeGen: soften f16 type by default instead of marking legal.

Actual support for softening f16 operations is still limited, and can be added
when it's needed.  But Soften is much closer to being a useful thing to try
than keeping it Legal when no registers can actually hold such values.

Longer term, we probably want something between Soften and Promote semantics
for most targets, it'll be more efficient to promote the 4 basic operations to
f32 than libcall them.