[flang] Use SomeExpr rather than GenericExprWrapper in checkers

Use GetExpr in checker classes to get analyzed expressions out of parse
tree nodes. Don't assume that they are always present. Change the
utility functions in tools.h to use SomeExpr rather than
GenericExprWrapper.

Original-commit: flang-compiler/f18@594e30d462b0592804b28146a34c74f1d1e2712a
Reviewed-on: https://github.com/flang-compiler/f18/pull/422
Tree-same-pre-rewrite: false