Make NfiTypeToIrType return the sign of the type

This is needed by the callers so they know to use signext or zeroext
when manipulating the value
Rename BoxedType.{llvmType => irType}

This value is called irType everwhere else in the Scala code. The
llvmType name was a holdover from boxedTypes.json.
DatumToNfiConstant -> DatumToNfiConstantSuite

The source file for the test was misnamed. The class was already correct.