IR: Fix ConstantExpr::replaceUsesOfWithOnConstant()

Change `ConstantExpr` to follow the model the other constants are using:
only malloc a replacement if it's going to be used.  This fixes a subtle
bug where if an API user had used `ConstantExpr::get()` already to
create the replacement but hadn't given it any users, we'd delete the
replacement.

This relies on r216015 to thread `OnlyIfReduced` through
`ConstantExpr::getWithOperands()`.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216016 91177308-0d34-0410-b5e6-96231b3b80d8