ScalarEvolution: Fix handling of AddRecs in isKnownPredicate

ScalarEvolution::isKnownPredicate() can wrongly reduce a comparison
when both the LHS and RHS are SCEVAddRecExprs. This checks that both
LHS and RHS are guarded in the case when both are SCEVAddRecExprs.

The test case is against indvars because I could not find a way to
directly test SCEV.

Patch by Sanjay Patel!

llvm-svn: 209487