[AArch64] Custom Lower MULLH{S,U} for v16i8, v8i16, and v4i32

This patch adds a custom lowering for ISD::MULH{S,U} used on divide by
constant optimization (DAGCombiner::BuildSDIV and DAGCombiner::BuildUDIV).

New patterns for smull and umull are added, so AArch64ISD::{S,U}MULL
can be correctly lowered to smull2 and umull2.

Reviewed By: SjoerdMeijer

Differential Revision: https://reviews.llvm.org/D46009

llvm-svn: 331522