[SCEV] Remember blocks for which we know symbolic exit count but not exact

The old code didn't bother to memoize blocks for which exact exit count is not
known. As result, in situation when exact isn't known but symbolic is known, this
info was lost. This patch fixes the situation: now we memoize when symbolic is
known (exact always implies symbolic, so this is a strict superset of what was before).

Differential Revision: https://reviews.llvm.org/D139515
Reviewed By: nikic