[riscv] Use X0 for destination of VSETVLI instruction if result unused

If the GPR destination register of a VSETVLI instruction is unused, we can replace it with X0. This discards the result, and thus reduces register pressure.

Since after the core insertion/lowering algorithm has run, many user written VSETVLIs will have their GPR result unused (as VTYPE/VLEN is now explicitly read instead), this kicks in for most tests which involve a vsetvli intrinsic for fixed length vectorization. (vscale vectorization generally uses the GPR result to know how far to e.g. advance pointers in a loop and these uses are not removed.)  When inserting VSETVLIs to lower psuedos, we prefer the X0 form anyways.

Differential Revision: https://reviews.llvm.org/D124961

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