[RISCV] Add test case for shufflevector that gets scalarized. NFC

This shufflevector gets scalarized into a build_vector of extract_vector_elts
because the output type doesn't match the input vector type.

Normally this is combined back into a vector_shuffle in DAGCombine, but this
one fails because we don't consider a extract_subvector to be cheap,
specifically because it's at an index > 31.

This should be canonicalized back into a vector_shuffle at some point so we can
lower it as a vrgather.vv.