rtl-reader: Disable reuse_rtx support for generator building

reuse_rtx is not documented nor the format to use it is ever documented.
So it should not be supported for the .md files.

This also fixes the problem if an invalid index is supplied for reuse_rtx,
instead of ICEing, put out a real error message.  Note since this code
still uses atoi, an invalid index can still be used in some cases but that is
recorded as part of PR 44574.

Note I did a grep of the sources to make sure that this was only used for
the read rtl in the GCC rather than while reading in .md files.

Bootstrapped and tested on x86_64-linux-gnu.

gcc/ChangeLog:

	* read-md.h (class rtx_reader): Don't include m_reuse_rtx_by_id
	when GENERATOR_FILE is defined.
	* read-rtl.cc (rtx_reader::read_rtx_code): Disable reuse_rtx
	support when GENERATOR_FILE is defined.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>