IR: Introduce ConstantAggregate, NFC

Add a common parent class for ConstantArray, ConstantVector, and
ConstantStruct called ConstantAggregate.  These are the aggregate
subclasses of Constant that take operands.

This is mainly a cleanup, adding common `isa` target and removing
duplicated code.  However, it also simplifies caching which constants
point transitively at `GlobalValue` (a possible future direction).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@265466 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed