SIL: Add an option to print all the cond_fail message strings encountered during IRGenPrepare

Run the swift-frontend in a mode that will generate LLVM IR adding the
option `-print-cond-fail-messages` will dump all cond_fail message strings
encountered in the SIL.

```
% swift-frontend -Xllvm -print-cond-fail-messages -emit-ir/-c ...
cond_fail message encountered: Can't take a prefix of negative length from a collection
cond_fail message encountered: Range out of bounds
cond_fail message encountered: Array index is out of range
```

Author: aschwaighofer

lib/SILOptimizer/Mandatory/IRGenPrepare.cpp

@@ -33,6 +33,13 @@
 
 using namespace swift;
 
+// Print the message string of encountered `cond_fail` instructions the first
+// time the message string is encountered.
+static llvm::cl::opt<bool> PrintCondFailMessages(
+  "print-cond-fail-messages", llvm::cl::init(false),
+  llvm::cl::desc("print cond_fail messages"));
+static llvm::DenseSet<StringRef> CondFailMessages;
+
 static bool cleanFunction(SILFunction &fn) {
   bool madeChange = false;
 
@@ -43,6 +50,26 @@ static bool cleanFunction(SILFunction &fn) {
       SILInstruction *inst = &*i;
       ++i;
 
+      // Print cond_fail messages the first time a specific cond_fail message
+      // string is encountered.
+      //   Run the swift-frontend in a mode that will generate LLVM IR adding
+      //   the option `-print-cond-fail-messages` will dump all cond_fail
+      //   message strings encountered in the SIL.
+      //   ```
+      //     % swift-frontend -Xllvm -print-cond-fail-messages -emit-ir/-c ...
+      //     ...
+      //     cond_fail message encountered: Range out of bounds
+      //     cond_fail message encountered: Array index is out of range
+      //     ...
+      //   ```
+      if (PrintCondFailMessages) {
+        if (auto CFI = dyn_cast<CondFailInst>(inst)) {
+          auto msg = CFI->getMessage();
+          if (CondFailMessages.insert(msg).second)
+            llvm::dbgs() << "cond_fail message encountered: " << msg << "\n";
+        }
+      }
+
       // Remove calls to Builtin.poundAssert() and Builtin.staticReport().
       auto *bi = dyn_cast<BuiltinInst>(inst);
       if (!bi) {