[X86] Don't pass a 1 to the second argument of ISD::FP_ROUND in LowerFCOPYSIGN.

I don't think we have any reason to believe the FP_ROUND here doesn't change the value.

Found while trying to see if we still need the fp128 block in CanCombineFCOPYSIGN_EXTEND_ROUND.
Removing that check caused this FP_ROUND to fire for fp128 which introduced a libcall expansion that asserted for this being a 1.

Reviewed By: RKSimon, pengfei

Differential Revision: https://reviews.llvm.org/D96098