Fix const-eval of vector init-lists of a vector.

Like any other type, an init list for a vector can have the same type as
the vector itself; handle that case.

<rdar://problem/14990460>
Don't build extra init lists.

AssignConvertType::IncompatibleVectors means the two types are in fact
compatible. :)

No testcase; I don't think the extra init list has any actual visible effect
other than making the resulting AST dump look a bit strange.