[RISCV] Teach lowerVECTOR_SHUFFLE to recognize some shuffles as vnsrl.

Unary shuffles such as <0,2,4,6,8,10,12,14> or <1,3,5,7,9,11,13,15>
where half the elements are returned, can be lowered using vnsrl.

SelectionDAGBuilder lowers such shuffles as a build_vector of
extract_elements since the mask has less elements than the source.
To fix this, I've enable the extractSubvectorIsCheapHook to allow
DAGCombine to rebuild the shuffle using 2 extract_subvectors preceding
the shufffle.

I've gone very conservative on extractSubvectorIsCheapHook to minimize
test impact and match what we have test coverage for. This can be
improved in the future.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D133736