[RISCV] Use LiveIntervals::extendToIndices to extend AVL in insertVSETVLI (#97512)
In #96200 we handled extending AVL LiveIntervals across basic blocks,
which fixed a crash in a test case in
133ab9a8c82a31549f060da33fd7e14f1d7f39fd.
This was done by manually adding a single segment to the LiveInterval to
extend it from AVL def -> inserted vsetvli, but in hindsight this was
too simple and fails to handle cases where the vsetlvi is located before
the AVL def.
This patch fixes this by using LiveIntervals::extendToIndices instead
which can handle these cases.
(The crash that this fixes is separate from the crash in #97264)