target-mips: gen_compute_branch1()

Optimize code generation in gen_compute_branch1():
- Directly use I32 variables instead of converting values from _tl to
  _i32 and back to _tl.
- Write the result directly to bcond instead of passing by a local
  variable.
- 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@5684 c046a42c-6fe2-441c-8c8c-71466251a162
