[InstCombine][ValueTracking] When computing known bits for Srem make sure we don...

[InstCombine][ValueTracking] When computing known bits for Srem make sure we don't compute known bits for the LHS twice.

If we already called computeKnownBits for the RHS being a constant power of 2, we've already computed everything we can and should just stop. I think previously we would still recurse if we had determined the result was negative or had not determined the sign bit at all.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@300432 91177308-0d34-0410-b5e6-96231b3b80d8