[RegisterCoalescer] Refine the terminal rule to still consider the terminal

[RegisterCoalescer] Refine the terminal rule to still consider the terminal
nodes.
When a node is terminal it is pushed at the end of the list of the copies to
coalesce instead of being completely ignored. In effect, this reduces its
priority over non-terminal nodes.

Because of that, we do not miss the rematerialization opportunities, nor the
copies that can be merged with more complex, than the terminal rule,
interference checks.

Related to PR22768.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233395 91177308-0d34-0410-b5e6-96231b3b80d8