[APFloat] prevent NaN morphing into Inf on conversion (PR43907)
We shift the significand right on a truncation, but that needs to be made NaN-safe:
always set at least 1 bit in the significand.
See D88238 for the likely follow-up (but needs some plumbing fixes before it can proceed).
Differential Revision: https://reviews.llvm.org/D87835