target-mips: variable names consistency

Use a consistent naming of arguments and TCG variables across the whole
file, the same as in tcg/tcg-op.h:
- arg1, arg2, ... for arguments
- t0, t1, t2, ... for variables

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

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@7106 c046a42c-6fe2-441c-8c8c-71466251a162