[flang][hlfir] lower hlfir.get_extent to FIR

By the ConvertToFIR pass, the hlfir.get_shape operation will have been
lowered into a fir.shape operation (during the HFLIR bufferization pass)
and so, lowering get_extent is as simple as fetching the extent from the
shape operation.

Depends on: D146833

Differential Revision: https://reviews.llvm.org/D148222