[llvm] [GlobalISel] Fix buildCopyFromRegs for split vectors (PR #77448)

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

Indeed v3bf16 was broken, I fixed it.
I imported all DAGISel tests to GISel and commented out the ones that don't work. GFX11 as a whole has trouble on some tests with truncs right now.

That way it's easier to see what doesn't work by just grepping FIXME in that file. Some cases should be easy to fix, I might look at it a bit to see how far I can go (unless someone else's already on it?)

https://github.com/llvm/llvm-project/pull/77448