[RISCV][InsertVSETVLI] Mutate prior vsetvli AVL if doing so allows us to remove a toggle

This extends the backwards walk to allow mutating the previous vsetvl's AVL value if it was not used by any instructions in between. In practice, this mostly benefits vmv.x.s and fvmv.f.s patterns since vector instructions which ignore VL are rare.

Differential Revision: https://reviews.llvm.org/D140048