Improving coverpoint quality of FD extensions.

[pawks]

• Coverpoints to test nan boxing behaviour (hand coded tests in most cases) - These coverpoints will mostly be described on the nan_prefix described in point 2 above. The minimal required bin is the walking1s.
• F Memory operations test - Memory ops should propagate NaN payloads in case of incorrect NaN boxing too. These will need specific scenarios and test cases. The coverpoints will also have to capture these scenarios precisely. The same coverpoints will also be applicable for the fmv instructions. ​
• Misaligned tests for F memory ops - The floating point memory ops require coverpoints and tests which check for the misaligned effective addresses like their integer counter parts. ​