Simplify the code in fmpz/fdiv_qr.c
Simplify the code in fmpz/fdiv_qr.c

This patch moves the three occurrences of two calls to
_fmpz_demote() for f and s into a single occurrences outside
the if or else blocks, since they are needed in any case.

fredrik-johansson/flint2,jpflori/flint2,wbhart/flint2,jpflori/flint2,jpflori/flint2,wbhart/flint2,dsroche/flint2,jpflori/flint2,fredrik-johansson/flint2,dsroche/flint2,fredrik-johansson/flint2,wbhart/flint2,dsroche/flint2,dsroche/flint2