[llvm] [AMDGPU][ISel] Set trunc store action to expand for v4f32->v4bf16 (PR #90427)

[Shilei Tian via llvm-commits]

shiltian wrote:

> The produced code is huge IMO, given the simplicity of the operation, but this is not a problem of the vector expand action itself.

How can we improve it?

https://github.com/llvm/llvm-project/pull/90427