[RISCV] Don't create insert/extract subreg during lowering. (#109754)

Create the equivalent INSERT_SUBVECTOR/EXTRACT_SUBVECTOR instead.

When we tried porting this to global isel, we noticed that subreg
operations are created early. We aren't able to do this until
instruction selection in global isel.

For SelectionDAG, it makes sense to use insert/extract_subvector as the
canonical form for these operations pre-isel. If it had come into
SelectionDAG as a insert/extract_subvector we would have kept it in that
form.

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