[FastISel][AArch64] Don't bail during simple GEP instruction selection.

The generic FastISel code would bail, because it can't emit a sign-extend for
AArch64. This copies the code over and uses AArch64 specific emit functions.

This is not ideal and 'computeAddress' should handles this, so it can fold the
address computation into the memory operation.

I plan to clean up 'computeAddress' anyways, so I will add that in a future
commit.

Related to rdar://problem/18962471.