[LAA] Do not set CanDoRT to false for AS that do not need RT checks.

Alternative approach to D80570.

canCheckPtrAtRT already contains checks the figure out for which alias
sets runtime checks are needed. But it currently sets CanDoRT to false
for alias sets for which we cannot do RT checks but also do not need
any.

If we know that we do not need RT checks based on the number of
reads/writes in the alias set, we can skip processing the AS.

This patch also adds an assertion to ensure that DepCands does not
contain more than one write from the alias set.

Reviewers: Ayal, anemet, hfinkel, dmgreen

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D80622