Fix disappearing shape when evaluating mask conditions in WHERE.

Fix indices assert.
Fix vector subscript lowering bug.
[cleanup] Eliminate call to SAEL.
Fix tests.
Fixes d7b and l6 end-to-end.

Remove the traversal boilerplate for scalar expressions that were lifted
to array context. This eliminates a few TODO items. It also merges all
array expression lowering (by context) into a single framework for
traversing evaluate Expr value trees. Both array and scalar expressions
in FORALLs require very similar lowering to FIR.

Author: schweitzpgi