target-mips: optimize gen_farith()

Optimize code generation in gen_farith():
- Temp variables are valid up to and *including* the brcond instruction.
  Use them instead of temp local variables.

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

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5682 c046a42c-6fe2-441c-8c8c-71466251a162
