[AArch64] Convert mul x, -(pow2 +/- 1) to shift + add/sub.

The combine for mul x, pow2 +/- 1 is unchanged. Test cases for
both combines as well as mul x, pow2 have been added as well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212044 91177308-0d34-0410-b5e6-96231b3b80d8