Interpret some constant expressions, fixes #305

[chriselrod]

I'll add more accurate determination of register pressure in another commit, probably to this same PR. The associated issue for that is this comment on Zulip.

For now, it just contains the interpretation of some constant expressions, which can sometimes drastically reduce the amount of arguments passed to _turbo_!.

[chriselrod]

Oh, and add vfmaddsub support, which I apparently haven't actually released in VectorizationBase yet.

[codecov]

Codecov Report

Merging #306 (1fc3177) into master (95ba84d) will decrease coverage by 0.76%.
The diff coverage is 86.18%.

@@            Coverage Diff             @@
##           master     #306      +/-   ##
==========================================
- Coverage   89.51%   88.74%   -0.77%     
==========================================
  Files          37       37              
  Lines        8057     8184     +127     
==========================================
+ Hits         7212     7263      +51     
- Misses        845      921      +76     

Impacted Files	Coverage Δ
src/LoopVectorization.jl	100.00% <ø> (ø)
src/modeling/costs.jl	67.58% <ø> (-0.47%)	⬇️
src/modeling/operations.jl	90.69% <0.00%> (-1.43%)	⬇️
src/codegen/lower_constant.jl	72.91% <38.63%> (-10.20%)	⬇️
src/modeling/graphs.jl	87.59% <82.25%> (-0.95%)	⬇️
src/codegen/loopstartstopmanager.jl	87.92% <87.50%> (-0.79%)	⬇️
src/parse/add_compute.jl	96.83% <93.33%> (-0.41%)	⬇️
src/codegen/lowering.jl	89.29% <95.23%> (-2.23%)	⬇️
src/modeling/determinestrategy.jl	96.14% <96.47%> (+0.02%)	⬆️
src/codegen/lower_store.jl	94.47% <100.00%> (-0.06%)	⬇️
... and 15 more

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