mips32: Fix the problem with the floating point compare instruction on mips32.

This patch is fixing the problem with emitting Iop_CmpF64.
Problem was introduced while running Valgrind for mips with v8 javascript engine.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2825 8f6e269a-dfd6-0310-a8e1-e2731360e62c