[sil] Add a cache for SILVerifier::isOperandInValueUses.
While looking at the performance of the verifier running with -sil-verify-all on
the stdlib, I noticed that we are spending ~30% of the total time in the
verifier performing this check. Introducing the cache mitigates this issue.
I believe the reason is that we were walking for each operand the use list of
its associated value which I think is quadratic.
