Add safety check to InstCombiner::commonIRemTransforms

Since FoldOpIntoPhi speculates the binary operation to potentially each
of the predecessors of the PHI node (pulling it out of arbitrary control
dependence in the process), we can FoldOpIntoPhi only if we know the
operation doesn't have UB.

This also brings up an interesting profitability question -- the way it
is written today, commonIRemTransforms will hoist out work from
dynamically dead code into code that will execute at runtime.  Perhaps
that isn't the best canonicalization?

Fixes PR27968.

llvm-svn: 271857