[mlir][memref] Revert #140730 (#141406)

Reverts #140730 - that turned out not to be an NFC as we originally
thought. See the attached test for an example. Many thanks to @Garra1980
for reporting!

Note, without this change, the newly added test would be incorrectly
converted to:
```mlir
  func.func @view_memref_as_rank0(%arg0: index, %arg1: memref<2xi8>) {
    %0 = llvm.mlir.poison : !llvm.struct<(ptr, ptr, i64)>
    return
  }
```

Author: banach-space

mlir/lib/Conversion/MemRefToLLVM/MemRefToLLVM.cpp

@@ -1721,10 +1721,6 @@ struct ViewOpLowering : public ConvertOpToLLVMPattern<memref::ViewOp> {
     MemRefDescriptor sourceMemRef(adaptor.getSource());
     auto targetMemRef = MemRefDescriptor::poison(rewriter, loc, targetDescTy);
 
-    // Early exit for 0-D corner case.
-    if (viewMemRefType.getRank() == 0)
-      return rewriter.replaceOp(viewOp, {targetMemRef}), success();
-
     // Field 1: Copy the allocated pointer, used for malloc/free.
     Value allocatedPtr = sourceMemRef.allocatedPtr(rewriter, loc);
     auto srcMemRefType = cast<MemRefType>(viewOp.getSource().getType());
@@ -1747,6 +1743,10 @@ struct ViewOpLowering : public ConvertOpToLLVMPattern<memref::ViewOp> {
         rewriter, loc,
         createIndexAttrConstant(rewriter, loc, indexType, offset));
 
+    // Early exit for 0-D corner case.
+    if (viewMemRefType.getRank() == 0)
+      return rewriter.replaceOp(viewOp, {targetMemRef}), success();
+
     // Fields 4 and 5: Update sizes and strides.
     Value stride = nullptr, nextSize = nullptr;
     for (int i = viewMemRefType.getRank() - 1; i >= 0; --i) {

mlir/test/Conversion/MemRefToLLVM/memref-to-llvm.mlir

@@ -1,10 +1,13 @@
-// RUN: mlir-opt -finalize-memref-to-llvm %s -split-input-file | FileCheck %s
+// RUN: mlir-opt -finalize-memref-to-llvm %s -split-input-file | FileCheck --check-prefixes=ALL,CHECK %s
 // RUN: mlir-opt -finalize-memref-to-llvm='index-bitwidth=32' %s -split-input-file | FileCheck --check-prefix=CHECK32 %s
 
 // Same below, but using the `ConvertToLLVMPatternInterface` entry point
 // and the generic `convert-to-llvm` pass. This produces slightly different IR
 // because the conversion target is set up differently.
-// RUN: mlir-opt --convert-to-llvm="filter-dialects=memref" --split-input-file %s | FileCheck --check-prefix=CHECK-INTERFACE %s
+// RUN: mlir-opt --convert-to-llvm="filter-dialects=memref" --split-input-file %s | FileCheck --check-prefixes=ALL,CHECK-INTERFACE %s
+
+// TODO: In some (all?) cases, CHECK and CHECK-INTERFACE outputs are identical.
+// Use a common prefix instead (e.g. ALL).
 
 // CHECK-LABEL: func @view(
 // CHECK: %[[ARG0F:.*]]: index, %[[ARG1F:.*]]: index, %[[ARG2F:.*]]: index
@@ -132,6 +135,28 @@ func.func @view_empty_memref(%offset: index, %mem: memref<0xi8>) {
 
 // -----
 
+// ALL-LABEL:   func.func @view_memref_as_rank0(
+// ALL-SAME:      %[[OFFSET:.*]]: index,
+// ALL-SAME:      %[[MEM:.*]]: memref<2xi8>) {
+func.func @view_memref_as_rank0(%offset: index, %mem: memref<2xi8>) {
+
+  // ALL:  builtin.unrealized_conversion_cast %[[OFFSET]] : index to i64
+  // ALL:  builtin.unrealized_conversion_cast %[[MEM]] : memref<2xi8> to !llvm.struct<(ptr, ptr, i64, array<1 x i64>, array<1 x i64>)>
+  // ALL:  llvm.mlir.poison : !llvm.struct<(ptr, ptr, i64)>
+  // ALL:  llvm.extractvalue %{{.*}}[0] : !llvm.struct<(ptr, ptr, i64, array<1 x i64>, array<1 x i64>)>
+  // ALL:  llvm.insertvalue %{{.*}}, %{{.*}}[0] : !llvm.struct<(ptr, ptr, i64)>
+  // ALL:  llvm.extractvalue %{{.*}}[1] : !llvm.struct<(ptr, ptr, i64, array<1 x i64>, array<1 x i64>)>
+  // ALL:  llvm.getelementptr %{{.*}}[%{{.*}}] : (!llvm.ptr, i64) -> !llvm.ptr, i8
+  // ALL:  llvm.insertvalue %{{.*}}, %{{.*}}[1] : !llvm.struct<(ptr, ptr, i64)>
+  // ALL:  llvm.mlir.constant(0 : index) : i64
+  // ALL:  llvm.insertvalue %{{.*}}, %{{.*}}[2] : !llvm.struct<(ptr, ptr, i64)>
+  %memref_view_bf16 = memref.view %mem[%offset][] : memref<2xi8> to memref<bf16>
+
+  return
+}
+
+// -----
+
 // Subviews needs to be expanded outside of the memref-to-llvm pass.
 // CHECK-LABEL: func @subview(
 // CHECK:         %[[MEMREF:.*]]: memref<{{.*}}>,