Improve handling of duplicate slotnames

[pfitzseb]

Turns out that we can sometimes end up with duplicate slotnames, which breaks the current logic in prepare_slotfunction.

E.g.

function debug(x)
    for iter in Iterators.CartesianIndices(x)
        i = iter[1]
        c = i
        a, b, c, d = tmp()
    end
    return x
end

results in

julia> c = @code_lowered debug((1,2,3));

julia> c.slotnames
10-element Vector{Symbol}:
 Symbol("#self#")
 :x
 Symbol("")
 Symbol("")
 :iter
 :d
 :b
 :a
 :c
 :i

which means that we never insert the proper assignment for i (because Symbol("") is in there twice).

[codecov]

Codecov Report

Merging #486 (b71da5a) into master (8e7bac9) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #486      +/-   ##
==========================================
- Coverage   86.94%   86.90%   -0.05%     
==========================================
  Files          12       12              
  Lines        2344     2336       -8     
==========================================
- Hits         2038     2030       -8     
  Misses        306      306              

Impacted Files	Coverage Δ
src/breakpoints.jl	95.37% <100.00%> (-0.21%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8e7bac9...b71da5a. Read the comment docs.

[KristofferC]

Looks like CI want some special handling for Julia 1.0.