[AArch64] Change bound for known zero bits of uaddlv intrinsic

As @efriedma's comment, the largest number of bits that can actually be set
for a v8i8 is 11 (the number of bits set in 8*255) so we can change the bound.
Additionally, v16i8 type is supported as v8i8.

Differential Revision: https://reviews.llvm.org/D158613