[CodeMetrics] Don't require speculatability for ephemeral values

As discussed in D112016, our current requirement of speculatability
for ephemeral is overly strict: What we really care about is that
the instruction will be DCEd once the assume is dropped. For that
it is sufficient that the instruction is side-effect free and not
a terminator.

In particular, this allows non-dereferenceable loads to be ephemeral
values.

Differential Revision: https://reviews.llvm.org/D112179