When a new phi is generated for scalarpre of an expression, the phiTranslate cache will become stale: Before PRE, the candidate expression must not be available in a predecessor block, and phitranslate will cache the information. After PRE, the expression will become available in all predecessor blocks, so the related entry in phiTranslate cache becomes stale. The patch will simply remove the stale entry so phiTranslate can be recomputed next time.
The stale entry in phitranslate cache will not affect correctness but will cause missing PRE opportunity for later instructions.