[IndVars] Fix a bug introduced in r317012

Turns out we can have comparisons which are indirect users of the induction variable that we can make invariant.  In this case, there is no loop invariant value contributing and we'd fail an assert.

The test case was found by a java fuzzer and reduced.  It's a real cornercase.  You have to have a static loop which we've already proven only executes once, but haven't broken the backedge on, and an inner phi whose result can be constant folded by SCEV using exit count reasoning but not proven by isKnownPredicate.  To my knowledge, only the fuzzer has hit this case.

llvm-svn: 319583