[FastISel][AArch64] Fix a missing nullptr check in 'computeAddress'.

The load/store value type is currently not available when lowering the memcpy
intrinsic. Add the missing nullptr check to support this in 'computeAddress'.

Fixes rdar://problem/19178947.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223818 91177308-0d34-0410-b5e6-96231b3b80d8