[FastISel][AArch64] Don't fold instructions that are not in the same basic block.

This fix checks first if the instruction to be folded (e.g. sign-/zero-extend,
or shift) is in the same machine basic block as the instruction we are folding
into.

Not doing so can result in incorrect code, because the value might not be
live-out of the basic block, where the value is defined.

This fixes rdar://problem/18169495.