mips32/64: Fix the problem with dirty helper when the
__mips_isa_rev is not 2 on higher.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2770 8f6e269a-dfd6-0310-a8e1-e2731360e62c
mips32/64: Code cleanup and VEX optimizations. No functional changes.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2771 8f6e269a-dfd6-0310-a8e1-e2731360e62c