[AARCH64] Enable STORE of v4i8 to help more vectorization opportunities

For the attached test case, currently llvm generates instructions to load/or/store the bytes one by one. Although NEON doesn't support v4i8 natively, we can promote it to v4i16 and operate on v4i16 vectors. So this patch override getStoreMinimumVF and specify the minimum VF for i8 vector is v4i8.

Differential Revision: https://reviews.llvm.org/D145614