[mlir][sparse] avoid single small vector, set exact number 3

[aartbik]

need vector for API

[wrengr]

LGTM (modulo the buildbot failures)

[aartbik]

In D138071#3929464, @wrengr wrote:

LGTM (modulo the buildbot failures)

codegen_buffer_initialization.mlir has been failing non-deterministically on Windows ;-(

mlir/lib/Dialect/SparseTensor/Transforms/SparseBufferRewriting.cpp

static void createPartitionFunc(OpBuilder &builder, ModuleOp module,
   Value lo = args[loIdx];
   Value hi = args[hiIdx];
   Value sum = builder.create<arith::AddIOp>(loc, lo, hi);
   Value c1 = constantIndex(builder, loc, 1);
   Value p = builder.create<arith::ShRUIOp>(loc, sum, c1);
 
   Value i = lo;
   Value j = builder.create<arith::SubIOp>(loc, hi, c1);
-  SmallVector<Value> operands{i, j, p};
-  SmallVector<Type> types{i.getType(), j.getType(), p.getType()};
+  SmallVector<Value, 3> operands{i, j, p};  // exactly three
+  SmallVector<Type, 3> types{i.getType(), j.getType(), p.getType()};
   scf::WhileOp whileOp = builder.create<scf::WhileOp>(loc, types, operands);
 
   // The before-region of the WhileOp.
   Block *before =
       builder.createBlock(&whileOp.getBefore(), {}, types, {loc, loc, loc});
   builder.setInsertionPointToEnd(before);
   Value cond = builder.create<arith::CmpIOp>(loc, arith::CmpIPredicate::ult,
                                              before->getArgument(0),

mlir/lib/Dialect/SparseTensor/Transforms/SparseTensorCodegen.cpp

if (isCompressedDim(rtp, d)) {
       if (d > 0) {
         unsigned ptrWidth = getSparseTensorEncoding(rtp).getPointerBitWidth();
         Type indexType = builder.getIndexType();
         Type ptrType = ptrWidth ? builder.getIntegerType(ptrWidth) : indexType;
         Value mz = constantIndex(builder, loc, getMemSizesIndex(field));
         Value hi = genLoad(builder, loc, fields[memSizesIdx], mz);
         Value zero = constantIndex(builder, loc, 0);
         Value one = constantIndex(builder, loc, 1);
-        SmallVector<Value, 1> inits;  // only one
-        inits.push_back(genLoad(builder, loc, fields[field], zero));
+        // Vector of only one, but needed by createFor's prototype.
+        SmallVector<Value, 1> inits{genLoad(builder, loc, fields[field], zero)};
         scf::ForOp loop = createFor(builder, loc, hi, inits, one);
         Value i = loop.getInductionVar();
         Value oldv = loop.getRegionIterArg(0);
         Value newv = genLoad(builder, loc, fields[field], i);
         Value ptrZero = constantZero(builder, loc, ptrType);
         Value cond = builder.create<arith::CmpIOp>(
             loc, arith::CmpIPredicate::eq, newv, ptrZero);
         scf::IfOp ifOp = builder.create<scf::IfOp>(loc, TypeRange(ptrType),