[flang] Deallocate WHERE masks after all assignments

Allocatable assignments were triggering lowering to clean-up
any WHERE mask temporaries, even if some assignments where left
in the WHERE construct.

This is because allocatable assignments lowering was being passed the
wrong statement context. Fix this by selecting the where/forall statement
context instead of a local one when there is one.

Differential Revision: https://reviews.llvm.org/D134197