Convert Vec dynamic index with a literal to static index

[jackkoenig]

This reduces the amount of work that both Chisel and firtool have to do.

Contributor Checklist

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• Performance improvement

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[jackkoenig]

@aswaterman has reminded me that we actually support out-of-bounds indices currently which means this could cause some currently working code to become an error. I'm going to change this to not accidentally change semantics and open a separate PR making those out-of-bounds indices a warning à la #3033.