[X86] Simplify the lowering code for extracting and inserting subvectors.

We don't need to check if AVX is enabled. It's implied by the operation action being set to Custom.
We don't need to check both the input and output type widths. We only need to check the type that's being inserted or extracted. The other type is known to be a legal type and we can assume its a different width.

llvm-svn: 284102