[flang][hlfir] get extents from hlfir.shape_of

If the extents were known, this should have been canonicalised into a
fir.shape operation. Therefore, the extents at this point are not known at
compile time. Use hlfir.get_extents to delay resolving the real extent
until after the expression is bufferized.

Depends On: D146831

Differential Revision: https://reviews.llvm.org/D146832