NaN-Boxing tests

[davidharrishmc]

D should have a basic 64-bit NaN-boxing of 32-bit operands test for every .s instruction checking:

• Well-NaN-boxed inputs
• Badly NaN-boxed on each input
  Also D should test flw for the various corners of single-precision numbers to ensure they are NaN-boxed into double-precision registers.

Similarly, Zf should have the same tests for 32-bit boxing of 16-bit operands, and ZfD should have the same test for 64-bit boxing of 16-bit operands. Both should test flh across corners.

[coreyqh]

Additionally, current improperly NaNBoxed argument coverpoints and tests only exercise fs1, not fs2 or fs3 where applicable.