[msan] Propagate shadow through (x<0) and (x>=0) comparisons.

[msan] Propagate shadow through (x<0) and (x>=0) comparisons.

This is a special case of signed relational comparison where result
only depends on the sign of x.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@168881 91177308-0d34-0410-b5e6-96231b3b80d8