[simplify-cfg] Add option sil-simplify-cfg-simplify-unconditional-branches for testing purposes

I am going to use this in a subsequent commit to make sure we do not infinite
loop upon a test case.

rdar://55861081
(cherry picked from commit fab7752)

Author: gottesmm

lib/SILOptimizer/Transforms/SimplifyCFG.cpp

@@ -1209,9 +1209,17 @@ static bool isReachable(SILBasicBlock *Block) {
 }
 #endif
 
+static llvm::cl::opt<bool> SimplifyUnconditionalBranches(
+    "simplify-cfg-simplify-unconditional-branches", llvm::cl::init(true));
+
 /// simplifyBranchBlock - Simplify a basic block that ends with an unconditional
 /// branch.
 bool SimplifyCFG::simplifyBranchBlock(BranchInst *BI) {
+  // If we are asked to not simplify unconditional branches (for testing
+  // purposes), exit early.
+  if (!SimplifyUnconditionalBranches)
+    return false;
+
   // First simplify instructions generating branch operands since that
   // can expose CFG simplifications.
   bool Simplified = simplifyBranchOperands(BI->getArgs());