[mlir] Apply ClangTidy fixes.

- Prefer to check empty() instead of size() == 0.
- Remove unused using declarations.

Author: akuegel

mlir/lib/Dialect/SCF/Transforms/ForallToFor.cpp

@@ -23,8 +23,6 @@ namespace mlir {
 
 using namespace llvm;
 using namespace mlir;
-using scf::ForallOp;
-using scf::ForOp;
 using scf::LoopNest;
 
 LogicalResult

mlir/lib/Dialect/SCF/Transforms/ParallelLoopFusion.cpp

@@ -85,7 +85,7 @@ static bool haveNoReadsAfterWriteExceptSameIndex(
       return WalkResult::advance();
 
     // Check that at last one store was retrieved
-    if (!write->second.size())
+    if (write->second.empty())
       return WalkResult::interrupt();
 
     auto storeIndices = write->second.front();