[SelectionDAG] Add FP_TO_UINT_SAT/FP_TO_SINT_SAT to computeKnownBits/computeNumSignBits.

These nodes should saturate to their saturating VT. We can use this
information to know the bits past the VT are all zeros or all sign bits.

I think we might only have test coverage for the unsigned case. I'll
verify and add tests.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D116870