Remove setPreservesCFG from instcombine.  The pass, in particular, does not
preserve LoopSimplify because instcombine may replace branch predicates
with undef which loop simplify then replaces with always exit.  Replace
setPreservesCFG with the more constrained preservation of DomTree and
LoopInfo.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221223 91177308-0d34-0410-b5e6-96231b3b80d8