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

[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.

llvm-svn: 300432