fix #7197, backport a fix to SimplifyDemandedUseBits in LLVM
try disabling the vectorizer when the instcombine bug is present, to
see if it fixes the tests on travis.
Merge pull request #7313 from JuliaLang/jb/instcombine_patch

fix #7197, backport a fix to SimplifyDemandedUseBits in LLVM