[RISCV][InsertVSETVLI] Split out demanded property for zero/non-zero of VL

[RISCV][InsertVSETVLI] Split out demanded property for zero/non-zero of VL

The scalar move instructions (vmv.s.x, and fvmv.s.f) depend solely on whether the VL is 0 or non-zero. By tracking the fact we only demand the zeroness and not the whole VL value, we can allow changing VL over a scalar move. This helps to eliminate vsetvli toggles.

Differential Revision: https://reviews.llvm.org/D140157