InstCombine: Don't just copy known bits from the first operand of an srem.

That's obviously wrong. Conservatively restrict it to the sign bit, which
matches the original intention of this analysis. Fixes PR15940.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181518 91177308-0d34-0410-b5e6-96231b3b80d8