[ExpandIRInsts] Test fptoi expansion for small types
Allow testing fptoui/fptosi on half types, which are small enough
for alive2 to verify the result.
They currently pass for non-undef/poison input. (The fptoui
expansion is the same as fptosi, which is confusing, but not
incorrect, because the saturation it performs is not actually
required by fptoi.)
