IR: Fix ConstantArray::replaceUsesOfWithOnConstant()

Previously, `ConstantArray::replaceUsesOfWithOnConstant()` neglected to
check whether it becomes a `ConstantDataArray`.  Call
`ConstantArray::getImpl()` to check for that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215965 91177308-0d34-0410-b5e6-96231b3b80d8