[DAG] Fix Memory ordering check in ReduceLoadOpStore.

When merging through a TokenFactor we need to check that the
load may be ordered such that no other aliasing memory operations may
happen. It is not sufficient to just check that the load is a member
of the chain token factor as it there may be a indirect chain. Require
the load's chain has only one use.

This fixes PR37826.

Reviewers: spatel, davide, efriedma, craig.topper, RKSimon

Subscribers: hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D49388

llvm-svn: 337560