[RISCV] Add cost model tests for llvm.vector.{insert,extract}. NFC

For llvm.vector.extract, this tests combinations of inserting at a zero and
non-zero index, and extracting from a fixed or scalable vector.

For llvm.vector.insert, this tests the same combinations as extracts but with
an additional configuration for an undef vector. This is because we can use a
subregister insert if the index is 0 and the vector is undef, which should be
free.