During inter-procedural SCCP, also infer attributes on arguments, not
just return values. This allows other non-interprocedural passes to make
use of the information later.
Similarly to the existing range attribute inference, also infer the
nonnull attribute on function return values.
I think in practice FunctionAttrs will handle nearly all cases, the main
one I think it doesn't is cases involving branch conditions. But as we
already have the information here, we may as well materialize it.
IPSCCP can currently return worse results than SCCP for arguments that
are tracked interprocedurally, because information from attributes is
not used for them.
Fix this by intersecting in the attribute information when propagating
lattice values from calls.
Add NotConstant(Null) roots for nonnull arguments and then propagate
them through nuw/inbounds GEPs.
Having this functionality in SCCP is useful because it allows reliably
eliminating null comparisons, independently of how deeply nested they
are in selects/phis. This handles cases that would hit a cutoff in
ValueTracking otherwise.
The implementation is something of a MVP, there are a number of obvious
extensions (e.g. allocas are also non-null).
