[llvm] [ExpandLargeFpConvert] Support bfloat. (PR #87619)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> 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.

GlobalIsel can't support/represent bfloat right now. We aren't correctly falling back in some situations though, so I wouldn't worry about it for this patch 

https://github.com/llvm/llvm-project/pull/87619