[SCEV][NFC] Win some compile time from mass forgetMemoizedResults

Mass forgetMemoizedResults can be done more efficiently than bunch
of individual invocations of helper because we can traverse maps being
updated just once, rather than doing this for each invidivual SCEV.

Should be NFC and supposedly improves compile time.

Differential Revision: https://reviews.llvm.org/D112294
Reviewed By: reames