[RISCVInsertVSETVLI] Handle vl-preserve case in backwards rewrite

This updates the backwards mutation code to handle the case where the previous vset was in vl-preserving (x0, x0) form, but that VL was never used before the next vset which changes the VL. Since this requires writing both VL operands, eliminate the restriction on removing GPR producing vsetv as well. (The register will now be written by the earlier vsetv.)

Differential Revision: https://reviews.llvm.org/D158019