[AArch64] When combining constant mul of power of 2 plus/minus 1, prefer shift
plus add.  The shift can be folded into the add.  This only effects codegen
when the constant is 3.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210445 91177308-0d34-0410-b5e6-96231b3b80d8