Avoid spurious warnings in pass skipping

[kripken]

Nested runners should be ignored, as they run some internal stuff in
certain passes, which would not contain the pass the user asked to
skip with --skip-pass.

Diff without whitespace is trivial.

[tlively]

I could see this being somewhat confusing for passes like inlining that run an entire optimization pipeline in a nested pass runner, but I don't know that it would be worth the extra complexity to "fix" that.

[kripken]

Good point, this would miss the nested full pipeline run of wasm-opt --inlining-optimizing. But yeah, not worth it to fix I think.