[llvm] [GlobalISel] Add G_INSERT_SUBVECTOR to computeNumSignBits (PR #220976)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

Any chance you can pull out the SignBitsOps::insertSubvector + SelectionDAG change into its own pre-PR to keep it separate from the GISel changes?

https://github.com/llvm/llvm-project/pull/220976