target-mips: optimize gen_arith()/gen_arith_imm()

Optimize code generation in gen_arith()/gen_arith_imm():
- Don't do sign extension when the value is already guaranteed to be
  sign extended (otherwise, results are marked as UNPREDICTABLE).
- When the value is sign extended, compare the value to 0 instead of
  testing bit 31/63.
- 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@5680 c046a42c-6fe2-441c-8c8c-71466251a162
