[mlir][VectorOps] Redo the scalar loop emission in VectoToSCF to pad instead of clipping

This replaces the select chain for edge-padding with an scf.if that
performs the memory operation when the index is in bounds and uses the
pad value when it's not. For transfer_write the same mechanism is used,
skipping the store when the index is out of bounds.

The integration test has a bunch of cases of how I believe this should
work.

Differential Revision: https://reviews.llvm.org/D87241

mlir/lib/Conversion/VectorToLLVM/ConvertVectorToLLVM.cpp

@@ -1096,7 +1096,7 @@ static bool isContiguous(MemRefType memRefType,
                          SmallVectorImpl<int64_t> &strides) {
   int64_t offset;
   auto successStrides = getStridesAndOffset(memRefType, strides, offset);
-  bool isContiguous = (strides.back() == 1);
+  bool isContiguous = strides.empty() || strides.back() == 1;
   if (isContiguous) {
     auto sizes = memRefType.getShape();
     for (int index = 0, e = strides.size() - 2; index < e; ++index) {