[RISCV] Mark subvector extracts from index 0 as cheap (#134101)

Previously we only marked fixed length vector extracts as cheap, so this
extends it to any extract at index 0 which should just be a subreg
extract.

This allows extracts of i1 vectors to be considered for DAG combines,
but also scalable vectors too.

This causes some slight improvements with large legalized fixed-length
vectors, but the underlying motiviation for this is to actually prevent
an unprofitable DAG combine on a scalable vector in an upcoming patch.

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