IR: Move NumElements field from {Array,Vector}Type to SequentialType.

IR: Move NumElements field from {Array,Vector}Type to SequentialType.

Now that PointerType is no longer a SequentialType, all SequentialTypes
have an associated number of elements, so we can move that information to
the base class, allowing for a number of simplifications.

Differential Revision: https://reviews.llvm.org/D27122

llvm-svn: 288464