Only use the even/odd/thirds lets for whole-vector extracts

[abadams]

rewrite_interleavings gives each vector let a pair of extra lets holding its even and odd lanes (or three holding every third lane). Those hold half or a third as many lanes as the let they come from, so a deinterleave asking for any other number of lanes must not be rewritten to use them.

visit(Shuffle) reaches exactly that case: when it extracts a single lane it narrows starting_lane but leaves lane_stride and new_lanes alone, so recursing into a vector of a different width leaves the two out of sync. The result was a variable bound to a value of the wrong type.

This path is apparently not reachable directly from front-end code, but it's a latent bug that popped up when I started messing with the simplifier.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.05%. Comparing base (bb5426d) to head (65b6b0b).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9363      +/-   ##
==========================================
- Coverage   70.08%   70.05%   -0.03%     
==========================================
  Files         259      259              
  Lines       79158    79160       +2     
  Branches    19293    19294       +1     
==========================================
- Hits        55477    55458      -19     
- Misses      17886    17906      +20     
- Partials     5795     5796       +1     

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