LoadError on simple example

I'm struggling to understand why this function won't load:

```
using LoopVectorization
function f(X::Matrix{Float64})
retval = [1]
@turbo for i ∈ eachindex(X)
retval[1] += X[i]
end
end
```

When I try to execute this code in Julia 1.6.2--not execute the f() function--I get:
```
ERROR: LoadError: LoadError: KeyError: key Symbol("###RHS###5###") not found
Stacktrace:
[1] getindex
@ .\dict.jl:482 [inlined]
[2] add_reduction!(ls::LoopVectorization.LoopSet, var::Symbol, reduceddeps::Vector{Symbol}, deps::Vector{Symbol}, vparents::Vector{LoopVectorization.Operation}, reduction_ind::Int64, elementbytes::Int64, instr::LoopVectorization.Instruction)
```

Reproduce the provided @turbo reduction example on Julia 1.6.2, starting with the load-time stack trace and LoopVectorization's reduction handling. Done means the example loads without the reported KeyError; the payload names no specific source file or test to run.

由索引模型根据 Issue 内容生成。