[CopyPropagation] Eliminate redundant moves.
SemanticARCOpts already eliminates move values that are redundant that
block its optimizations. But it's always run after CopyPropagation.
Because move_values divide copy-extended lifetimes, move_values obstruct
lifetime canonicalization. If a move_value isn't separating lifetimes
with different characteristics (specifically: lexicallity, escaping),
then it is only obstructing lifetime canonicalization. Remove it
before canonicalizing the lifetime of the moved-from value.
