[LCSSA] Don't invalidate entire loop in SCEV

We already invalidate each individual instruction for which LCSSA
is formed in formLCSSAForInstructions(), so I don't see a reason
why we would need to invalidate the entire loop on top of that.

I believe we also no longer need the instruction-level invalidation
now that SCEV looks through LCSSA phis, but I'll leave that for a
separate patch, as it's less obvious.

Differential Revision: https://reviews.llvm.org/D149331