MIPSr6 has max.s/max.d/min.s/min.d instructions, which can be used as
fcanonicalize.
For pre-R6, we have no instructions that can fcanonicalize an float, so
let's use `fadd Y,X,X` to quiet it if it is NaN.
IEEE754-2008 requires that the result of general-computational and
quiet-computational operation shouldn't be signal NaN.