DAG: Relax foldBitcastedFPLogic conditions

Requiring a bitcast to exist was unhelpful. The most basic cases
are always going to be a CopyFromReg or load, so they would need
a new cast inserted. Don't require a bitcast if it's a free
operation. I don't think this logic makes particularly much sense
(it seems to be imparting special interpretation of bitcast), but
this needs to be in sync with foldSignChangeInBitcast.

We should also get rid of this hasBitPreservingFPLogic hook. fabs/fneg
are bitpreserving or incorrectly implemented, so this should just be a
regular legality check.