[flang] Reorganize evaluate::Assignment

Every analyzed assignment represented by `evaluate::Assignment` has
a LHS and RHS expression. These need to be checked uniformly in various
places. So change Assignment always to have those data members, with
the variant determining which kinds of assignment it is: intrinsic,
user-defined, or pointer.

Original-commit: flang-compiler/f18@fb87d16a868112e26ade0ad696b2232d8cf3a524
Reviewed-on: https://github.com/flang-compiler/f18/pull/989
Tree-same-pre-rewrite: false