set sample_element's probeInfo for Vecs

[debs-sifive]

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• Bugfix

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For Vecs, also set the probeInfo of their sample_element, which is used to determine its element type.

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[azidar]

While I approve, i'd like to see a test case that covers this error.

[dtzSiFive]

Can you add a test to check this works as intended and continues to? Thanks!

[debs-sifive]

Working on it! Accidentally merged too early, so it will be coming in a separate PR.

[debs-sifive]

OK actually after talking with Adam more, the sample_element issue no longer affects the FIRRTL that gets emitted — we were only seeing it before with the probe aggregate-blasting approach because that required probe modifiers to get nested inside of Vecs, which no longer happens/is legal to get emitted.

So it's not something we can detect anymore in the emitted firrtl... that said, I think we should keep this fix in place because it makes sense for the sample_element to have the same probe information to the actual vector elements in case it gets used down the line. Which I know is not going to be super satisfying b/c of this whole "source of truth" on the vector element type getting mutated in this random place :(