[GuardWidening] Widen guards with conditions of frequently taken dominated branches

If there is a frequently taken branch dominated by a guard, and its condition is available
at the point of the guard, we can widen guard with condition of this branch and convert
the branch into unconditional:

  guard(cond1)
  if (cond2) {
    // taken in 99.9% cases
    // do something
  } else {
    // do something else    
  }

Converts to

  guard(cond1 && cond2)
  // do something

Differential Revision: https://reviews.llvm.org/D49974
Reviewed By: reames

llvm-svn: 338988