LTO: Get rid of 'lto_mode_identity_table'

This, in particular, resolves LTO ICEs with big 'machine_mode's, as for RISC-V.
('mode_table' in 'lto_file_decl_data' still is 'unsigned char'; changing that
is still to be done (for use in offloading compilation), but is not trivial.)
For now, get rid of 'lto_mode_identity_table' to resolve the RISC-V LTO ICEs;
we don't need an actual table for a 1-to-1 mapping.

	gcc/lto/
	* lto-common.cc (lto_mode_identity_table): Remove.
	(lto_file_finalize) [!ACCEL_COMPILER]: 'NULL'-intialize
	'file_data->mode_table'.
	(lto_fe_init): Don't initialize 'lto_mode_identity_table'.
	* lto-common.h (lto_mode_identity_table): Remove.
	gcc/
	* tree-streamer.h (bp_unpack_machine_mode): If
	'ib->file_data->mode_table' not available, apply 1-to-1 mapping.

Co-authored-by: Pan Li <pan2.li@intel.com>