Correct description of short-circuit evaluation

Add that final value in a conditional chain may be anything and provide an example
Return NaN for median of vectors containing NaN. Closes #6486
Merge pull request #6819 from simonp0420/patch-1

Correct description of short-circuit evaluation
Merge pull request #6820 from JuliaLang/sjk/median_nan

Return NaN for median of vectors containing NaN. Closes #6486
Add SLPVectorizer, and patch with enhancements for vectorizing tuples.
The patch contains functionality and tests backported from recent
SLPVectorizer changes added to LLVM trunk.
Removing trailing space in order to make Travis retry the build.