[FastISel][AArch64] Followup commit for 218031 to handle negative offsets too.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218032 91177308-0d34-0410-b5e6-96231b3b80d8
[FastISel][AArch64] Simplify XALU multiplies.

Simplify {s|u}mul.with.overflow to {s|u}add.with.overflow when possible.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218033 91177308-0d34-0410-b5e6-96231b3b80d8