`Broadcast.broadcast_shape` for BlockedUnitRanges fails inference

MWE:
```julia
using BlockArrays
shape1 = (BlockArrays._BlockedUnitRange((2,)),);
shape2 = (BlockArrays._BlockedUnitRange((2,)),);
Base.Broadcast._bcs(shape1, shape2)
# @code_warntype Base.Broadcast._bcs1(shape1[1], shape2[1]) # lower level, but internals
@code_warntype Base.Broadcast.broadcast_shape(shape1, shape2)
```