[FastIsel][AArch64] Fix a think-o in address computation.

When looking through sign/zero-extensions the code would always assume there is
such an extension instruction and use the wrong operand for the address.

There was also a minor issue in the handling of 'AND' instructions. I
accidentially used a 'cast' instead of a 'dyn_cast'.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218161 91177308-0d34-0410-b5e6-96231b3b80d8