[SelectionDAG] Rewrite bfloat16 softening to use the "half promotion" path

The main difference is that this preserves intermediate rounding steps,
which the other route doesn't. This aligns bfloat16 more with half
floats, which use this path on most targets.

I didn't understand what the difference was between these softening
approaches when I first added bfloat lowerings, would be nice if we only
had one of them.

Based on @pengfei 's D131502

Differential Revision: https://reviews.llvm.org/D133207