Instead of testing whether an instruction dominates the loop preheader,

Instead of testing whether an instruction dominates the loop preheader,
test whether it properly dominates the loop header. This is equivalent
when the loop has a preheader, and has the advantage of working when
the loop doesn't have a preheader. Since IVUsers doesn't Require
LoopSimplify, the loop isn't guaranteed to have a preheader.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@82899 91177308-0d34-0410-b5e6-96231b3b80d8