[X86][AVX] Cleanup gather_v8i32_v8i32 special test case

Cleanup the gather_v8i32_v8i32 IR to more closely match how the middle-end will optimise the vector geps (exposing more splats).

This helps the gather scalarization case a lot, but shows a missed opportunity for AVX512 gathers to recognise uniform-constant indices.

And none of the cases realise that some of the gathers are really blended broadcasts....