Prior to this commit, `NaN` values did not have any special handling
in either `StructuralEqual` or `StructuralHash`.

`StructuralEqual` checked whether the LHS and RHS were within some
tolerance of each other.  If the LHS and RHS are both `NaN`, this
would evaluate to false.  The updated `StructuralEqual` now checks for
this case, and returns true if both sides are `NaN`.

`StructuralHash` used the bit-pattern of a floating-point number to
compute the hash.  A `NaN` value may have any non-zero value in its
mantissa, and so this could produce distinct hashes for ASTs that
differ only by the choice of non-zero value.  The updated
`StructuralHash` uses the same
`std::numeric_limits<double::quiet_NaN()` value for all `NaN` values.

With these changes, `StructuralEqual` and `StructuralHash` can now
compare two IR functions, even if they contain `NaN`.

Closes apache#17247