[SCEV] Turn validity check in getExistingSCEV into assert (NFC).

Now that we track users of SCEV expressions, we should be able to always
invalidate containing expressions.

With that, I think the case where a value gets removed but
SCEVs containing references to it should not be possible any longer.
Turn check into an assert.

This slightly reduces compile-time:

NewPM-O3: -0.27%
NewPM-ReleaseThinLTO: -0.21%
NewPM-ReleaseLTO-g: -0.26%

http://llvm-compile-time-tracker.com/compare.php?from=c3dc6b081da6ba503e67d260033f81f61eb38ea3&to=95a4a028b1f1dd0bc3d221435953b7d2c031b3d5&stat=instructions

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D114633