[mlir] Allow empty position in vector.insert and vector.extract

Such ops are no-ops and are folded to their respective `source`/`vector` operand.

Differential Revision: https://reviews.llvm.org/D101879

mlir/lib/Conversion/VectorToLLVM/ConvertVectorToLLVM.cpp

@@ -656,6 +656,12 @@ public:
     if (!llvmResultType)
       return failure();
 
+    // Extract entire vector. Should be handled by folder, but just to be safe.
+    if (positionArrayAttr.empty()) {
+      rewriter.replaceOp(extractOp, adaptor.vector());
+      return success();
+    }
+
     // One-shot extraction of vector from array (only requires extractvalue).
     if (resultType.isa<VectorType>()) {
       Value extracted = rewriter.create<LLVM::ExtractValueOp>(
@@ -762,6 +768,13 @@ public:
     if (!llvmResultType)
       return failure();
 
+    // Overwrite entire vector with value. Should be handled by folder, but
+    // just to be safe.
+    if (positionArrayAttr.empty()) {
+      rewriter.replaceOp(insertOp, adaptor.source());
+      return success();
+    }
+
     // One-shot insertion of a vector into an array (only requires insertvalue).
     if (sourceType.isa<VectorType>()) {
       Value inserted = rewriter.create<LLVM::InsertValueOp>(