[AMDGPU] Vectorize more 16 bit shuffles (#90648)
In the case of larger vectors, we should still prefer the vectorized
version (i.e. shufflevector vs extract/insert chains).
In arithmetic chains, vectorization results in chains of packed math
instructions (as opposed to unpack/repack & scalarized arithmetic):
https://godbolt.org/z/c5onaf6G5
In chains with PHIs, vectorization again removes the unnecessary pack /
repack code around BBs: https://godbolt.org/z/vz7zYzvhs
