CodeGen: Optimize emssion of zeroinitialzied arrays

Create an ConstantAggregateZero upfront if we see that it is viable.
This saves us from having to manually push_back each and every
initializer and then looping back over them to determine if they are
'null'.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224908 91177308-0d34-0410-b5e6-96231b3b80d8