B4 and B5 coverage missing converts

[coreyqh]

Covergroups presently only require arithmetic operation coverage.

Some converts can cause underflow/underflow, so we need to cover those cases.

[sww0000]

The covergroup for B4 also has some impossible cases, such as operations like OP_SQRT and other sign negation operations that would need to be adjusted (I thought I posted an issue about this on Tuesday, but I guess it didn't get opened for some reason...?)

[sww0000]

Oh nvm, it has been resolved.