[LV] Relax over-strict assertion for reduction exit value selects.

After f108c6cd, (mul x, 1) is simplified to x, which can cause the select
for the final reduction value when tail-folding to use the reduction
value for both options. Relax the assertion to make sure this case is
allowed.

Note that the reduction is now redundant itself and could be further
simplified.

Fixes #66895.