[X86] Change vXi1 extract_vector_elt lowering to be legal if the index is 0. Add DAG combine to turn scalar_to_vector+extract_vector_elt into extract_subvector.

We were lowering the last step extract_vector_elt to a bitcast+truncate. Change it to use an extract_vector_elt of index 0 instead. Add isel patterns to do the equivalent of what the bitcast would have done. Plus an isel pattern for an any_extend+extract to prevent some regressions.

Finally add a DAG combine to turn v1i1 scalar_to_vector+extract_vector_elt of 0 into an extract_subvector.

This fixes some of the regressions from D350800.

llvm-svn: 350918

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