[IVDescriptors] Fix bug in checkOrderedReduction

The Exit instruction passed in for checking if it's an ordered reduction need not be
an FPAdd operation. We need to bail out at that point instead of
assuming it is an FPAdd (and hence has two operands). See added testcase.
It crashes without the patch because the Exit instruction is a phi with
exactly one operand.
This latent bug was exposed by 95346ba8 which added support for
multi-exit loops for vectorization.

Reviewed-By: kmclaughlin
Differential Revision: https://reviews.llvm.org/D106843