[SimplifyCFG] Add command-line option to disable the pass

The option is useful for some of the experiments we're doing.
I don't see any harm in having such option and the ability to disable
the pass.

Author: madhur13490

llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp

@@ -85,6 +85,10 @@ static cl::opt<bool> UserSpeculateUnpredictables(
     "speculate-unpredictables", cl::Hidden, cl::init(false),
     cl::desc("Speculate unpredictable branches (default = false)"));
 
+static cl::opt<bool> DisableSimplifyCFG(
+    "disable-simplify-cfg", cl::Hidden, cl::init(false),
+    cl::desc("Disable simplify cfg"));
+
 STATISTIC(NumSimpl, "Number of blocks simplified");
 
 static bool
@@ -307,6 +311,9 @@ static bool simplifyFunctionCFG(Function &F, const TargetTransformInfo &TTI,
           (DT && DT->verify(DominatorTree::VerificationLevel::Full))) &&
          "Original domtree is invalid?");
 
+  if (DisableSimplifyCFG)
+    return false;
+
   bool Changed = simplifyFunctionCFGImpl(F, TTI, DT, Options);
 
   assert((!RequireAndPreserveDomTree ||

llvm/test/Transforms/SimplifyCFG/disable-simplify-cfg.ll

@@ -0,0 +1,13 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt < %s -passes=simplifycfg -disable-simplify-cfg=true -S | FileCheck %s
+
+define void @test1() {
+; CHECK-LABEL: define void @test1() {
+; CHECK-NEXT:    br label %[[BB1:.*]]
+; CHECK:       [[BB1]]:
+; CHECK-NEXT:    ret void
+;
+  br label %1
+  ret void
+}
+