fix destruction of compound if blocks with outros

[Rich-Harris]

Fixes a bug I just saw in the REPL

[codecov-io]

Codecov Report

Merging #572 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #572      +/-   ##
==========================================
+ Coverage   94.45%   94.45%   +<.01%     
==========================================
  Files          95       95              
  Lines        2777     2779       +2     
==========================================
+ Hits         2623     2625       +2     
  Misses        154      154

Impacted Files	Coverage Δ
src/generators/dom/visitors/IfBlock.js	98.9% <100%> (+0.02%)	⬆️

---

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 35d820f...e8c07e8. Read the comment docs.