[mlir][llvm] Add assertion in LLVM IR import (NFC).

[gysit]

Summary

This revision adds an assertion to convertConstantExpr to ensure.
The function is not called if the constant has been converted before.

This is a follow up for https://reviews.llvm.org/D149247.

[gysit]

Remove debug left over.

[victor-eds]

LGTM, thanks!

mlir/lib/Target/LLVMIR/ModuleImport.cpp

FailureOr<Value> ModuleImport::convertConstant(llvm::Constant *constant) {
   StringRef error = "";
   if (isa<llvm::BlockAddress>(constant))
     error = " since blockaddress(...) is unsupported";
 
   return emitError(loc) << "unhandled constant: " << diag(*constant) << error;
 }
 
 FailureOr<Value> ModuleImport::convertConstantExpr(llvm::Constant *constant) {
+  // Only call the function for constants that have not been translated before
+  // since it updates the constant insertion point assuming the converted
+  // constant has been introduced at the end of the constant section.
+  assert(!valueMapping.contains(constant) &&
+         "expected constant has not been converted before");
   assert(constantInsertionBlock &&
          "expected the constant insertion block to be non-null");
 
-  // Insert the constant after the last one or at the start or the entry block.
+  // Insert the constant after the last one or at the start of the entry block.
   OpBuilder::InsertionGuard guard(builder);
   if (!constantInsertionOp)
     builder.setInsertionPointToStart(constantInsertionBlock);
   else
     builder.setInsertionPointAfter(constantInsertionOp);
 
   // Convert all constants of the expression and add them to `valueMapping`.
   SetVector<llvm::Constant *> constantsToConvert =