[RISCV][TTI] Cost a subvector extract at a register boundary with exact vlen (#82405)

If we have exact vlen knowledge, we can figure out which indices
correspond to register boundaries. Our lowering uses this knowledge to
replace the vslidedown.vi with a sub-register extract. Our costs can
reflect that as well.

This is another piece split off
https://github.com/llvm/llvm-project/pull/80164



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Co-authored-by: Luke Lau <luke_lau@icloud.com>