NVPTX: Use RAUW instead of reinventing the wheel

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.

llvm-svn: 215953