[DAG][AArch64] Handle vector types when expanding sdiv/udiv into mulh

The aarch64 backend will benefit from expanding 64vector sdiv/udiv into mulh
using shift(mul(ext, ext)), as the larger type size is legal and the mul(ext,
ext) can efficiently use smull/umull instructions. This extends the existing
code in GetMULHS to handle vector types for it.

Differential Revision: https://reviews.llvm.org/D154049