Introduce a new LoopInfo utility function makeLoopInvariant, which

Introduce a new LoopInfo utility function makeLoopInvariant, which
works similar to isLoopInvariant, except that it will do trivial
hoisting to try to make the value loop invariant if it isn't already.
This makes it easier for transformation passes to clear trivial
instructions out of the way (the regular LICM pass doesn't run
until relatively late). This is code factored out of LoopSimplify
and other places.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@75578 91177308-0d34-0410-b5e6-96231b3b80d8