pair-fusion: Add singleton move_range asserts [PR114492]

The PR claims that pair-fusion has invalid uses of gcc_assert (such that
the pass will misbehave with --disable-checking).  As noted in the
comments, in the case of the calls to restrict_movement, the only way we
can possibly depend on the side effects is if we call it with a
non-singleton move range.  However, the intent is that we always have a
singleton move range here, and thus we do not rely on the side effects.

This patch therefore adds asserts to check for a singleton move range
before calling restrict_movement, thus clarifying the intent and
hopefully dispelling any concerns that having the calls wrapped in
asserts is problematic here.

gcc/ChangeLog:

	PR rtl-optimization/114492
	* pair-fusion.cc (pair_fusion_bb_info::fuse_pair): Check for singleton
	move range before calling restrict_movement.
	(pair_fusion::try_promote_writeback): Likewise.