Proposal for adding rounding variant of fpto?i instructions

My high level thought on this is that it would be great to have, but is a major new feature. In addition to adding rounding mode selection to fpto?i, we should add it to all the rest of the fp operators. We also want to eventually add support for the optimizer to know that the user is messing around with rounding modes (which needs to disable inexact constant folding and other optimizations) as well. Modeling this properly will require some fairly substantial changes throughout the compiler similar to when NUW/NSW were added on the integer side.

-Chris