[flang][nfc] Add debug prints to FIR alias analysis

[tblah]

Summary

These make it easier to debug and improve alias analysis. Enable with
--debug-only=fir-alias-analysis.

[Renaud-K]

Thank you.

flang/lib/Optimizer/Analysis/AliasAnalysis.cpp

 #include "flang/Optimizer/Dialect/FIROpsSupport.h"
 #include "flang/Optimizer/Dialect/FIRType.h"
 #include "mlir/Analysis/AliasAnalysis.h"
 #include "mlir/IR/BuiltinOps.h"
 #include "mlir/IR/Value.h"
 #include "mlir/Interfaces/SideEffectInterfaces.h"
 #include "llvm/ADT/TypeSwitch.h"
 #include "llvm/Support/Casting.h"
+#include "llvm/Support/Debug.h"
 
 using namespace mlir;
 
+#define DEBUG_TYPE "fir-alias-analysis"
+
 //===----------------------------------------------------------------------===//
 // AliasAnalysis: alias
 //===----------------------------------------------------------------------===//
 
 static bool isDummyArgument(mlir::Value v) {
   auto blockArg{v.dyn_cast<mlir::BlockArgument>()};
   if (!blockArg)
     return false;
if (!eleTy)
     return false;
   // TO DO: Look for pointer components
   return eleTy.isa<fir::RecordType>();
 }
 
 AliasResult AliasAnalysis::alias(Value lhs, Value rhs) {
   auto lhsSrc = getSource(lhs);
   auto rhsSrc = getSource(rhs);
+  LLVM_DEBUG(llvm::dbgs() << "AliasAnalysis::alias\n";
+             llvm::dbgs() << "  lhs: " << lhs << "\n";
+             llvm::dbgs() << "  lhsSrc: " << lhsSrc << "\n";
+             llvm::dbgs() << "  rhs: " << rhs << "\n";
+             llvm::dbgs() << "  rhsSrc: " << rhsSrc << "\n";
+             llvm::dbgs() << "\n";);
 
   // Indirect case currently not handled. Conservatively assume
   // it aliases with everything
   if (lhsSrc.kind == SourceKind::Indirect ||
       lhsSrc.kind == SourceKind::Unknown ||
       rhsSrc.kind == SourceKind::Indirect || rhsSrc.kind == SourceKind::Unknown)
     return AliasResult::MayAlias;