[SCEV] Don't invalidate past dependency-breaking instructions

When invalidating a value, we walk all users of that value and
invalidate them as well. This can be very expensive for large use
graphs.

However, we only need to invalidate a user U of instruction I if
SCEV(U) can depend on SCEV(I). This is not the case if U is an
instruction that always produces a SCEVUnknown, such as a load.
If the load pointer operand is invalidated, there is no need to
invalidate the load result, which is completely unrelated from a
SCEV perspective.

Differential Revision: https://reviews.llvm.org/D149323