target-ppc: fix fres, fsqrte and remove useless code

- fres and fsqrte should not assign a float32 number to a float64 value.
- fre, fres and fsqrte are checking for cases already taken into account
  by softfloat and softfloat native. Remove those useless tests.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6083 c046a42c-6fe2-441c-8c8c-71466251a162
