[msan] Implement exact shadow propagation for relational ICmp.

Only for integers, pointers, and vectors of those. No floats.
Instrumentation seems very heavy, and may need to be replaced
with some approximation in the future.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173452 91177308-0d34-0410-b5e6-96231b3b80d8