Change default method of disabling JIT

https://wiki.php.net/rfc/jit_config_defaults
Closes GH-12678

Improve jit tests

Closes GH-12425

Fix return type verification with undef var

This was loading EG(uninitialized_value) into r0 rather than
FCARG1a.

However, if we fix this issue an existing test fails because

Fix return type verification with undef var

This was loading EG(uninitialized_value) into r0 rather than
FCARG1a.

However, if we fix this issue an existing test fails because
the undef var warning promoted to exception is not caught early
enough, so we need to explicitly check for the exception before
performing the type check.

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