[FastISel][AArch64] Fix sign-/zero-extend folding when SelectionDAG is involved.

Sign-/zero-extend folding depended on the load and the integer extend to be
both selected by FastISel. This cannot always be garantueed and SelectionDAG
might interfer. This commit adds additonal checks to load and integer extend
lowering to catch this.

Related to rdar://problem/18495928.