[X86][AVX] Fold extract_subvector(VSRLI/VSHLI(x,32)) -> VSRLI/VSHLI(extract_subvector(x),32)

As discussed on D56387, if we're shifting to extract the upper/lower half of a vXi64 vector then we're actually better off performing this at the subvector level as its very likely to fold into something.

combineConcatVectorOps can perform this in reverse if necessary.