[RISCV] Move AVL coalescing logic upwards into computeInfoForInstr. NFC (#73909)

There is an optimisation in transferBefore where if a VSETVLIInfo uses
the AVL
of a defining vsetvli, it uses that vsetvli's AVL provided VLMAX is the
same.

This patch moves it out of transferBefore and up into
computeInfoForInstr to
show how it isn't affected by the other optimisations in transferBefore,
and to
simplify the control flow by removing an early return.

This should make #72352 easier to reason about.