[RISCV] Unify getDemanded between forward and backwards passes in RISCVInsertVSETVLI (#92860)

We have two rules in needVSETVLI where we can relax the demanded fields
for slides and splats when VL=1.

However these aren't present in getDemanded which prevents us from
coalescing some vsetvlis around slides and splats in the backwards pass.

The reasoning as to why they weren't in getDemanded is that these
require us to check the value of the AVL operand, which may be stale in
the backwards pass: the actual VL or VTYPE value may differ from what
was precisely requested in the pseudo's operands.

Using the original operands should actually be fine though, as we only
care about what was originally demanded by the instruction. The current
value of VL or VTYPE shouldn't influence this.

This addresses some of the regressions we are seeing in #70549 from
splats and slides getting reordered.