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).

llvm-svn: 265466

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -1708,8 +1708,7 @@ static void WriteConstants(unsigned FirstVal, unsigned LastVal,
           Record.push_back(
               CDS->getElementAsAPFloat(i).bitcastToAPInt().getLimitedValue());
       }
-    } else if (isa<ConstantArray>(C) || isa<ConstantStruct>(C) ||
-               isa<ConstantVector>(C)) {
+    } else if (isa<ConstantAggregate>(C)) {
       Code = bitc::CST_CODE_AGGREGATE;
       for (const Value *Op : C->operands())
         Record.push_back(VE.getValueID(Op));