[ScopDetectDiagnostics] Do not format unnamed array names

Formatting unnamed array names is expensive in LLVM as the this requires
deriving the numbered virtual instruction name (e.g., %12) for an llvm::Value,
which is currently not implemented efficiently. As instruction numberes anyhow
do not really carry a lot of information for the user, we just print 'unknown'
instead.

This change reduces the scop detection time from 24 to 19 seconds, for one of
our large-scale inputs. This is a reduction by 21%.

llvm-svn: 294894

polly/lib/Analysis/ScopDetectionDiagnostic.cpp

@@ -443,7 +443,7 @@ std::string ReportAlias::formatInvalidAlias(std::string Prefix,
     assert(V && "Diagnostic info does not match found LLVM-IR anymore.");
 
     if (V->getName().size() == 0)
-      OS << "\"" << *V << "\"";
+      OS << "\" <unknown> \"";
     else
       OS << "\"" << V->getName() << "\"";