[fir] Add desc to fir.array_load op and update operand name

This patch is part of the upstreaming effort from fir-dev branch.

Add a description for the fir.array_load opeartion and rename lenParams to typeparams.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D110393



Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>