SIL: Allow to selectively disabled cond_fails by cond_fail message

The standard library uses `_precondition` calls which have a message argument.

Allow disabling the generated cond_fail by these message arguments.

For example:

 _precondition(source >= (0 as T), "Negative value is not representable")

Results in a `cond_fail "Negative value is not representable"`.

This commit allows for specifying a file that contains these messages on
each line.

/path/to/disable_cond_fails:
```
Negative value is not representable
Array index is out of range
```

The optimizer will remove these cond_fails if the swift frontend is invoked with
`-Xllvm -cond-fail-config-file=/path/to/disable_cond_fails`.

Author: aschwaighofer

lib/SILOptimizer/SILCombiner/SILCombine.cpp

@@ -43,7 +43,10 @@
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Statistic.h"
+#include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"
+#include <fstream>
+#include <set>
 
 using namespace swift;
 
@@ -690,3 +693,29 @@ void SwiftPassInvocation::eraseInstruction(SILInstruction *inst) {
     }
   }
 }
+
+static llvm::cl::opt<std::string> CondFailConfigFile(
+    "cond-fail-config-file", llvm::cl::init(""),
+    llvm::cl::desc("read the cond_fail message strings to elimimate from file"));
+
+static std::set<std::string> CondFailsToRemove;
+
+bool SILCombiner::shouldRemoveCondFail(CondFailInst &CFI) {
+  if (CondFailConfigFile.empty())
+    return false;
+
+  std::fstream fs(CondFailConfigFile);
+  if (!fs) {
+    llvm::errs() << "cannot cond_fail disablement config file\n";
+    exit(1);
+  }
+  if (CondFailsToRemove.empty()) {
+    std::string line;
+    while (std::getline(fs, line)) {
+      CondFailsToRemove.insert(line);
+    }
+    fs.close();
+  }
+  auto message = CFI.getMessage();
+  return CondFailsToRemove.find(message.str()) != CondFailsToRemove.end();
+}

lib/SILOptimizer/SILCombiner/SILCombiner.h

@@ -136,6 +136,8 @@ class SILCombiner :
 
   bool runOnFunction(SILFunction &F);
 
+  bool shouldRemoveCondFail(CondFailInst &);
+
   void clear() {
     Iteration = 0;
     Worklist.resetChecked();

lib/SILOptimizer/SILCombiner/SILCombinerMiscVisitors.cpp

@@ -803,6 +803,9 @@ SILInstruction *SILCombiner::visitCondFailInst(CondFailInst *CFI) {
   if (RemoveCondFails)
     return eraseInstFromFunction(*CFI);
 
+  if (shouldRemoveCondFail(*CFI))
+    return eraseInstFromFunction(*CFI);
+
   auto *I = dyn_cast<IntegerLiteralInst>(CFI->getOperand());
   if (!I)
     return nullptr;