Add more let peeling and rewrapping helpers

[abadams]

Peeling Lets or LetStmts into a vector of name/value pairs, doing
something to the body, and then rewrapping is a common pattern in the
compiler. Rewrapping conditionally was done by calling expr_uses_var on
the partially-rebuilt body once per let, which is quadratic.

Adds a Stmt overload of peel_lets, plus rewrap_used_lets and
rewrap_all_lets for Expr and Stmt. rewrap_used_lets gathers the names
the body mentions once and extends the set with the value of each let it
keeps, so it is n log(n) instead of n^2. It conservatively treats every name
mentioned as a possible reference to a peeled let, even where an inner
let shadows it.

There should be no functional changes here, but while working on it Claude
noticed that the peeled lets in the store collector in Deinterleave.cpp are
unused. It looks like it was supposed to take the lets by reference instead of
by value, but if you do that you get duplicates in the list. It is currently trying
to figure out if this is just dead code or if this masks a bug.

[codecov]

Codecov Report

❌ Patch coverage is 88.54167% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.85%. Comparing base (03a18bb) to head (90c1c4b).

Files with missing lines	Patch %	Lines
src/IROperator.cpp	87.50%	5 Missing ⚠️
src/LowerWarpShuffles.cpp	0.00%	4 Missing ⚠️
src/CodeGen_ARM.cpp	50.00%	1 Missing ⚠️
src/SplitTuples.cpp	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9319      +/-   ##
==========================================
+ Coverage   69.83%   69.85%   +0.02%     
==========================================
  Files         258      258              
  Lines       78210    78165      -45     
  Branches    19037    19006      -31     
==========================================
- Hits        54617    54604      -13     
+ Misses      17832    17793      -39     
- Partials     5761     5768       +7     

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