When LSR converts a branch on pre-inc IV into a branch on post-inc IV, the nowrap flags on the addition may no longer be valid. Previously, a poison result of the addition might have been ignored, in which case the program was well defined. After branching on the post-inc IV, we might be branching on poison, which is undefined behavior.
Fix this by discarding nowrap flags which are not present on the SCEV expression. Nowrap flags on the SCEV expression are proven by SCEV to always hold, independently of how the expression will be used. This is essentially the same fix we applied to IndVars LFTR, which also performs this kind of pre-inc to post-inc conversion.
I believe a similar problem can also exist for getelementptr inbounds, but I was not able to come up with a problematic test case. The inbounds case would have to be addressed in a differently anyway (as SCEV does not track this property).