[IndVars] Forget the SCEV when the instruction has been sunk.

In the past, the SCEV expression of the sunk instruction was not
forgetted. This led to the incorrect block dispositions after the
instruction be sunk.

Fixes https://github.com/llvm/llvm-project/issues/58662

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D137060