Revert "[RISCV][InsertVSETVLI] Make VL preserving vsetvli emission more explicit [nfc]"

This reverts commit 20fc8e8d.  As pointed out in review of the mentioned follow up patch, this gets the predicate wrong.  We need not simply VL being unchanged, but VLMAX being unchanged.  Given that the code structure I'd introduced here is simply confusing.