[VectorOps] Add lowering of vector.broadcast to LLVM IR

For example, a scalar broadcast

    %0 = vector.broadcast %x : f32 to vector<2xf32>
    return %0 : vector<2xf32>

which expands scalar x into vector [x,x] by lowering
to the following LLVM IR dialect to implement the
duplication over the leading dimension.

    %0 = llvm.mlir.undef : !llvm<"<2 x float>">
    %1 = llvm.mlir.constant(0 : index) : !llvm.i64
    %2 = llvm.insertelement %x, %0[%1 : !llvm.i64] : !llvm<"<2 x float>">
    %3 = llvm.shufflevector %2, %0 [0 : i32, 0 : i32] : !llvm<"<2 x float>">, !llvm<"<2 x float>">
    return %3 : vector<2xf32>

In the trailing dimensions, the operand is simply
"passed through", unless a more elaborate "stretch"
is required.

For example

    %0 = vector.broadcast %arg0 : vector<1xf32> to vector<4xf32>
    return %0 : vector<4xf32>

becomes

    %0 = llvm.mlir.undef : !llvm<"<4 x float>">
    %1 = llvm.mlir.constant(0 : index) : !llvm.i64
    %2 = llvm.extractelement %arg0[%1 : !llvm.i64] : !llvm<"<1 x float>">
    %3 = llvm.mlir.constant(0 : index) : !llvm.i64
    %4 = llvm.insertelement %2, %0[%3 : !llvm.i64] : !llvm<"<4 x float>">
    %5 = llvm.shufflevector %4, %0 [0 : i32, 0 : i32, 0 : i32, 0 : i32] : !llvm<"<4 x float>">, !llvm<"<4 x float>">
    llvm.return %5 : !llvm<"<4 x float>">

PiperOrigin-RevId: 284219926