This code had a homemade RAUW that was incorrect when a user was a
constant: instead of calling `replaceUsersWithOnConstant()` it would
incorrectly update the operand in-place, invalidating
`LLVMContextImpl::ExprConstants`. RAUW does the job better.
The ValueHandle that `GVMap` is holding onto needs to be removed first,
so this commit also removes each variable from the map on-the-fly.
Since deletions from `ExprConstants` use a linear search that compares
directly on the pointer value (instead of using the key), there isn't an
obvious way to expose this with a testcase.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215953 91177308-0d34-0410-b5e6-96231b3b80d8
6b863256e4