Reapply "[RISCV][TTI] Extract subvector at index zero is free (#81751)"

This reverts commit 834d11c2 which was
a revert of my 3a626937.  I had failed
to rebase after new tests added overnight by
fc0b67e1.

Original commit message follows:

Extracing a subvector at index zero corresponds to a type conversion and
possibly a subregister operation. We will not emit a vslidedown. As such,
they are free.

As an aside, it looks like we're not passing an index in for cases where
the subvec type is scalable. For at least index zero, we probably should be.

Revert "Revert "[RISCV][TTI] Extract subvector at index zero is free (#81751)""