[ExpandLargeFpConvert] Support bfloat. (#87619)

The conversion expansions did not properly handle bfloat types.

I'm not certain that these expansions are completely correct;
I don't have any experience with AMDGPU or the ability to run
anything to test it.

Note that it doesn't seem like AMDGPU with GlobalISel can
handle fptrunc of float to bfloat, which is needed for itofp.
I've omitted the GISEL run for the bfloat case.

This fixes #85379.