[X86] Remove an fp->int->fp domain crossing in LowerUINT_TO_FP_i64.

Summary: This unfortunately adds a move, but isn't that better than going to the int domain and back?

Reviewers: RKSimon

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D52134

llvm-svn: 342327