spirv-opt: Behave a bit better in the face of unknown instructions (#2487)

* opt/ir_loader: Don't silently drop unknown instructions on the floor

Currently, if spirv-opt sees an instruction it does not know, it will
silently ignore it and move to the next one. This changes it
to be an error, as dropping it on the floor is likely to generate
invalid SPIR-V output.

* opt/optimizer: Complain a bit louder for unexpected binary changes

If a binary change happens despite a pass saying that the binaries
should be identical, this is indicative of a bug in the pass itself.

This does not change behavior for it to be an error, but simply emits a warning in this case.