Handle moving operations in reductions

This either `UndefVarError`s or (with a reordering in lowering) produces the wrong answer:
```julia
function doubleadd_loop_avxtest(x,a)
s = zero(eltype(x))
@avx for i in 1:length(x)
s += 2*x[i] + a
end
return s
end
```
because it transforms this into something like
```julia
stemp = s + a
for i in ...
sreduct = 2*x[i] + stemp
end
```
Where
1. `sreduct` isn't being accumulated across the loop's iterations.
2. `a` (after being hoisted out) is missing a constant multiple of `length(x) / W` where `W` is the SIMD vector width.

I should address both of these issues.

Start with the @avx reduction and lowering paths, using the doubleadd_loop_avxtest example in the issue to reproduce the failure. Trace how the reduction and hoisted a are transformed; done means the reduction accumulates across iterations and the hoisted value includes the required multiple of length(x) / W.

Written by the indexing model from the issue text.