[LegalizeTypes] Remove PromoteFloat support form ExpandIntRes_LLROUND_LLRINT.

This code isn't exercised, and was in the wrong place. If we need
this, we would need to promote the type before figuring out which
libcall to use.

I'm choosing to remove it rather than fixing since we don't
support PromoteFloat for LRINT/LROUND/LLRINT/LLROUND when the
result type is legal so I don't see much reason to support it
for the case where the result type isn't legal.