[RegionPass] Fix forgotten "!".

Commit r355068 "Fix IR/Analysis layering issue with OptBisect" uses the
template

   return Gate.isEnabled() && !Gate.shouldRunPass(this, getDescription(...));

for all pass kinds. For the RegionPass, it left out the not operator,
causing region passes to be skipped as soon as a pass gate is used.

llvm-svn: 355733