IR: Reduce RAUW traffic in ConstantVector

Avoid creating a new `ConstantVector` on an RAUW of one of its members.
This reduces RAUW traffic on any containing constant.

This is part of PR20515.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215966 91177308-0d34-0410-b5e6-96231b3b80d8