[ome] Invoke simplifyInstruction after lowering ownership and use replaceAllSimplifiedUsesAndErase instead of a manual RAUW.

I am doing two things here:

1. simplifyInstruction expects its result to only be passed to
replaceAllSimplifiedUsesAndErase. This was an oversite.

2. I am upstreaming support for InstSimplify in OSSA (to finish sil-combine
work). The main assumption that inst-simplify is going to have is that it is
looking at SIL in correct ownership ssa or non-ownership ssa. It is not going to
be designed to work with an intermediate world that is the world where before
this PR it was being invoked (specifically, we have marked the function as not
being in OSSA but haven't finished transforming it into non-OSSA SIL). To avoid
this problem, I moved the simplification to the end of the pass after we have
done the initial traversal.

Should be NFCI.

Author: gottesmm