[RISCV] Add an early out to lowerVECTOR_SHUFFLEAsVSlidedown. NFC

If Mask[0] is 0, then we're never going to match a slidedown. If
we get through the for loop, then it's an identity mask which should
have already been optimized out. Otherwise it's some non-contiguous
mask that will fail out of the lop. Might as well not bother entering
the loop.