…if-true` argument narrowing

- When a function/method/static call carries `@phpstan-assert*` assertions, the
  call handlers previously returned only the assert-specified argument narrowing
  and skipped `handleDefaultTruthyOrFalseyContext`, so the call expression itself
  was not remembered as truthy/falsey in the branch. Re-evaluating it (e.g.
  `if (is_readable($p)) { require $p; }`) then yielded `bool` instead of `true`.
- `FuncCallHandler`, `MethodCallHandler` and `StaticCallHandler` now union the
  asserts result with the default truthy/falsey narrowing, while preserving the
  asserts' original root expression so impossible-check detection is unaffected.
- `ImpossibleCheckTypeHelper` now ignores the self-referential narrowing entry
  (the checked call expression itself), which carries no information about
  whether the check is redundant — the informative narrowing lives in the
  argument entries.
- `RequireFileExistsRule` additionally treats `is_readable`, `is_writable`,
  `is_writeable` and `is_executable` (next to `file_exists`/`is_file`) as guards
  that prove the path exists, via a named `FILE_EXISTENCE_FUNCTIONS` constant.
- Fixed a genuine `X && !X` bug in the build-only
  `OrChainIdenticalComparisonToInArrayRule::getSubjectAndValue()` that the
  improved narrowing newly detects.