[FastISel][AArch64] Fix load/store with frame indices.

At higher optimization levels the LLVM IR may contain more complex patterns for
loads/stores from/to frame indices. The 'computeAddress' function wasn't able to
handle this and triggered an assertion.

This fix extends the possible addressing modes for frame indices.

This fixes rdar://problem/18783298.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220700 91177308-0d34-0410-b5e6-96231b3b80d8