[X86][AVX] Combine 128-bit lane shuffles with a zeroable upper half to EXTRACT_SUBVECTOR (PR40720)

As explained on PR40720, EXTRACTF128 is always as good/better than VPERM2F128, and we can use the implicit zeroing of the upper half.

I've added some extra tests to vector-shuffle-combining-avx2.ll to make sure we don't lose coverage.