Add SLPVectorizer, and patch with enhancements for vectorizing tuples.
The patch contains functionality backported from recent SLPVectorizer
patches submitted to the LLVM community.
Merge branch 'adr/slpvector' of https://github.com/ArchRobison/julia into adr/slpvector
Avoid infinite retry problem discovered in SVN trunk version.
Add relevant test from LLVM trunk.