gdb/jit: apply some simplifications and assertions

Following patch "gdb/jit: split jit_objfile_data in two", there are some
simplifications we can make.  The invariants described there mean that
we can assume / assert some things instead of checking them using
conditionals.

If an instance of jiter_objfile_data exists for a given objfile, it's
because the required JIT interface symbols were found.  Therefore, in
~jiter_objfile_data, the `register_code` field can't be NULL.  It was
previously used to differentiate a jit_objfile_data object used for a
JITer vs a JITed.  We can remove that check.

If an instance of jiter_objfile_data exists for a given objfile, it's
because it's the sole JITer objfile in the scope of its program space
(jit_program_space_data::objfile points to it).  At the moment,
jit_breakpoint_re_set_internal won't create a second instance of
jiter_objfile_data for a given program space.  Therefore, it's not
necessary to check for `ps_data != NULL` in ~jiter_objfile_data: we know
a jit_program_space_data for that program space exists.  We also don't
need to check for `ps_data->objfile == this->objfile`, because we know
the objfile is the sole JITer in this program space.  Replace these two
conditions with assertions.

A pre-condition for calling the jit_read_descriptor function (which is
respected in the two call sites) is that the objfile `jiter` _is_ a
JITer - it already has a jiter_objfile_data attached to it.  When a
jiter_objfile_data exists, its `descriptor` field is necessarily set:
had the descriptor symbol not been found, jit_breakpoint_re_set_internal
would not have created the jiter_objfile_data.  Remove the check and
early return in jit_read_descriptor.  Access objfile's `jiter_data` field
directly instead of calling `get_jiter_objfile_data` (which creates the
jiter_objfile_data if it doesn't exist yet) and assert that the result
is not nullptr.

Finally, `jit_event_handler` is always passed a JITer objfile.  So, add
an assertion to ensure that.

gdb/ChangeLog:
2020-07-22  Simon Marchi  <simon.marchi@polymtl.ca>

	* jit.c (jiter_objfile_data::~jiter_objfile_data): Remove some
	checks.
	(jit_read_descriptor): Remove NULL check.
	(jit_event_handler): Add an assertion.

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