[LegacyPM] Call getPassName only when needed

Even when time tracing is disabled, getPassName is currently still
called. This adds an avoidable virtual function call for each pass.
Fetching the pass name only when required slightly improves
compile-time (particularly when LLVM is built without LTO).

Reviewed By: nikic, MaskRay

Differential Revision: https://reviews.llvm.org/D148022

llvm/lib/IR/LegacyPassManager.cpp

@@ -1414,7 +1414,10 @@ bool FPPassManager::runOnFunction(Function &F) {
     FunctionPass *FP = getContainedPass(Index);
     bool LocalChanged = false;
 
-    llvm::TimeTraceScope PassScope("RunPass", FP->getPassName());
+    // Call getPassName only when required. The call itself is fairly cheap, but
+    // still virtual and repeated calling adds unnecessary overhead.
+    llvm::TimeTraceScope PassScope(
+        "RunPass", [FP]() { return std::string(FP->getPassName()); });
 
     dumpPassInfo(FP, EXECUTION_MSG, ON_FUNCTION_MSG, F.getName());
     dumpRequiredSet(FP);