[FastISel][AArch64] Fold mul into the address computation of memory operations.

Teach 'computeAddress' to also fold multiplies into the address computation
(when possible).

This fixes rdar://problem/18369443.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217977 91177308-0d34-0410-b5e6-96231b3b80d8