[SimplifyCFG] If provided, preserve Dominator Tree

SimplifyCFG is an utility pass, and the fact that it does not
preserve DomTree's, forces it's users to somehow workaround that,
likely by not preserving DomTrees's themselves.

Indeed, simplifycfg pass didn't know how to preserve dominator tree,
it took me just under a month (starting with e113317)
do rectify that, now it fully knows how to,
there's likely some problems with that still,
but i've dealt with everything i can spot so far.

I think we now can flip the switch.

Note that this is functionally an NFC change,
since this doesn't change the users to pass in the DomTree,
that is a separate question.

Reviewed By: kuhar, nikic

Differential Revision: https://reviews.llvm.org/D94827

Author: LebedevRI

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -6561,20 +6561,10 @@ bool SimplifyCFGOpt::simplifyOnceImpl(BasicBlock *BB) {
 bool SimplifyCFGOpt::simplifyOnce(BasicBlock *BB) {
   bool Changed = simplifyOnceImpl(BB);
 
-  assert((!RequireAndPreserveDomTree ||
-          (DTU &&
-           DTU->getDomTree().verify(DominatorTree::VerificationLevel::Full))) &&
-         "Failed to maintain validity of domtree!");
-
   return Changed;
 }
 
 bool SimplifyCFGOpt::run(BasicBlock *BB) {
-  assert((!RequireAndPreserveDomTree ||
-          (DTU &&
-           DTU->getDomTree().verify(DominatorTree::VerificationLevel::Full))) &&
-         "Original domtree is invalid?");
-
   bool Changed = false;
 
   // Repeated simplify BB as long as resimplification is requested.
@@ -6592,7 +6582,7 @@ bool SimplifyCFGOpt::run(BasicBlock *BB) {
 bool llvm::simplifyCFG(BasicBlock *BB, const TargetTransformInfo &TTI,
                        DomTreeUpdater *DTU, const SimplifyCFGOptions &Options,
                        ArrayRef<WeakVH> LoopHeaders) {
-  return SimplifyCFGOpt(TTI, RequireAndPreserveDomTree ? DTU : nullptr,
-                        BB->getModule()->getDataLayout(), LoopHeaders, Options)
+  return SimplifyCFGOpt(TTI, DTU, BB->getModule()->getDataLayout(), LoopHeaders,
+                        Options)
       .run(BB);
 }