Fix ineffective if warnings in discarded case branches

[JSap0914]

What

• inspect each case branch's final expression when the case result is discarded
• report an ineffective final conditional even when earlier statements in that branch have side effects
• preserve value-producing branch expressions when the case result is assigned or otherwise used

Why

The validator only checked whether the entire case expression was idempotent. An earlier assignment made the case look productive, so an ineffective if at the end of the branch escaped the existing "value was produced and then forgotten" diagnostic.

Fixes #542

Tests

• bundle exec rspec spec/unit/pops/validator/validator_spec.rb: 359 examples, 0 failures
• bundle exec rspec spec/unit/pops: 4,798 examples, 0 failures, 3 expected pending examples
• bundle exec rubocop lib/puppet/pops/validation/checker4_0.rb: no offenses
• ruby -wc on both changed Ruby files: syntax OK