Previously copy propagation presumed that there is further unspecified
distinction between move operands and copy operands in assignments and
propagated moves from assignments into terminators. This is inconsistent
with current operational semantics.
Turn moves into copies after copy propagation to preserve existing behavior.
Fixes https://github.com/rust-lang/rust/issues/137936.
Fixes https://github.com/rust-lang/rust/issues/146423.