Add an accessor to ValueObject that determines if the object represents a base class, and also returns the depth of base-class-ness. For instance if one has class C : public B {} class B : public A {}, the value for A nested in B nested in C would be a base class of depth 2
[FastISel][AArch64] Factor out ADDS/SUBS instruction emission and add support for extensions and shift folding.

Factor out the ADDS/SUBS instruction emission code into helper functions and
make the helper functions more clever to support most of the different ADDS/SUBS
instructions the architecture support. This includes better immedediate support,
shift folding, and sign-/zero-extend folding.

This fixes <rdar://problem/17913111>.