[flang] Refactor Analyze(PointerAssignmentStmt)

Use early returns to reduce the indentation.
Check LHS is a pointer as early as possible.
A PointerAssignmentStmt can only have a typedAssignment that
represents a PointerAssignment. So assert that is the case and
don't worry about the other cases.

Original-commit: flang-compiler/f18@bdf3d3a292b5f9ce4241e3d63b759bd61838cf18
Reviewed-on: https://github.com/flang-compiler/f18/pull/944
Tree-same-pre-rewrite: false