[IRCE] Fix buggy behavior in Clamp

[IRCE] Fix buggy behavior in Clamp

Clamp function was too optimistic when choosing signed or unsigned min/max function for calculations.
In fact, `!IsSignedPredicate` guarantees us that `Smallest` and `Greatest` can be compared safely using unsigned
predicates, but we did not check this for `S` which can in theory be negative.

This patch makes Clamp use signed min/max for cases when it fails to prove `S` being non-negative,
and it adds a test where such situation may lead to incorrect conditions calculation.

Differential Revision: https://reviews.llvm.org/D36873

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@311205 91177308-0d34-0410-b5e6-96231b3b80d8