[llvm] [GISEL] Add G_INSERT_SUBVECTOR and G_EXTRACT_SUBVECTOR (PR #84538)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I think we need G_EXTRACT as the equivalent of EXTRACT_SUBREG in addition to G_INSERT_SUBVECTOR

Yes, it is the equivalent of EXTRACT_SUBREG, which is why it's problematic. It's essentially unlegalizable since it's too unrestricted

https://github.com/llvm/llvm-project/pull/84538